Bar Mitzvah-ed on a Different Day?
As detailed at length in recent articles, our current year, 5785, is full of rare calendarical phenomena.[1] This article sets out to describe an interesting one that can affect Bar Mitzvah bachurim - one that will ensure that they will observe their Bar Mitzvah this year not on their actual birthday, but actually a day earlier. Other years, this interesting divergence may cause Bar Mitzvah observance to occur a day after their actual birthday. Can this truly be? Indubitably so. Read on intrepid one…
30 Days Has November…
As is well known from Chazal, and actually millennia later corroborated by NASA,[2] the Lunar Month is 29 days, 12 hours and 793 chalakim (or 44 minutes and one cheilek; a cheilik equals three and one-third seconds, an 18th of a minute, or an 1080th of an hour). In the times of Chazal, based on eyewitness accounts of the New Moon, any month could have had 30 days (called a chodesh malei or full month) or 29 days (referred to as a chosesh chaseir). In our 19-year cycle Jewish calendar established by Hillel II (Hillel Nesiah; a thirteenth or fourteenth-generation descendant of Hillel Hazakein),[3] every other month, alternates between 29 and 30 days.
Whereas the Gregorian calendar maxim might be “Thirty Days has September, April, June, and November,” but in the Jewish calendar the colloquialism would probably be “Thirty Days has Tishrei, Shvat, Nissan, Sivan, and Av. Teves, Adar,[4] Iyar, Tamuz, and Elul only have 29 days.”[5]
An easy way to tell if the month you are in has 29 or 30 days is by seeing how many days of Rosh Chodesh the following month has. If it has two-days of Rosh Chodesh, that means the first day of Rosh Chodesh is actually the 30th day of the preceding month. For example, the first day of Rosh Chodesh Elul is actually Lamed Av. Even so, since it is connected to the month of Elul, there are halachos how the date is supposed to be written in official documents. Take our date of Lamed Av. Halachically it is to be written as “Rosh Chodesh Elul, Shehu Yom Shloshim L’Chodesh (which is the thirtieth of the month of) Av.”[6] On the other hand, if it is a one day Rosh Chodesh, it is simply the 1st of that month.
Wild Cards?
As you probably realized, there were two months missing from the “adage of the months.” Marcheshvan and Kislev were not mentioned. The reason is that there is no hard and fast rule regarding them. They are the only months that can either have 29 or 30 days depending on the year. Some years both are chaseirim; other years both are malei’im, and others Cheshvan is chaseir and Kislev malei.
The reasons and rules governing the whys and whens this transpires are too complicated for this discussion; suffice to say that when the Mishnah Berurah discusses these issues he writes “ain kan makom l’ha’arich,” that this is not the place to expound in detail,[7] which is certainly good enough for this author. What is a given, is that this is one of the changing variables in our set calendar. Indeed, it is this specific variable that allows a Bar Mitzvah to occur before or after the actual presumed Bar Mitzvah date.
In fact, “Mein Alte Shachein”, the esteemed Bobover Rebbe, Rav Shlomo Halberstam zt”l,[8] was known to have remarked that he personally started laying Tefillin as a Bar Mitzvah Bachur one day prior to his Bar Mitzvah.[9] His intent was not that the Bobov minhag was to prepare for a Bar Mitzvah by laying Tefillin only one day before the actual Bar Mitzvah, but rather that he himself was full fledge “Bar Mitzvah-ed” (in the vernacular) before the date of his Bar Mitzvah. The reason why is quite interesting, due to a cosmic “quirk” in our calendar.
Will the Real Adar Please Stand Up?
But, this is not the only time that a calendar quirk changes a Bar Mitzvah. It is widely known that adding a leap year into the mix always has interesting Bar Mitzvah ramifications. The majority consensus is that if a boy was born in a non-leap year, one which there was only one Adar, and on the year of his Bar Mitzvah there are two Adars, his Bar Mitzvah will occur in the second Adar, since it is considered the true one.[10] The same holds true if was actually born in Adar Sheini. In fact the only way one would celebrate a Bar Mitzvah in the first Adar is if he was actually born in an Adar Rishon.
This makes for a remarkable dichotomy. If one boy is born on the 21st of Adar Rishon, and his buddy a week and a half later on the 2nd of Adar Sheini, then in any standard year following, the second one would be celebrating his birthday almost 3 weeks before his “older” friend. Since there is only one Adar, the second born’s birthday would be the 2nd of Adar while his “older” friend’s would be on the 21st. In fact, only in a leap year would the older one truly be considered older. This would also affect their Bar Mitzvahs. If their Bar Mitzvah is in a standard year, the younger lad would become a man several weeks before his older compadre.[11] However, it is important to note that this Klal may not hold true regarding Yahrtzeits.[12]
Rare Year
Back to Cheshvan and Kislev. As detailed in recent articles, 5785 is not only a rare year, but calendarically speaking, actually the hands-downrarest of them all. This is because it is classified as a HaSh”A year in our calendars. This abbreviation is referring to Rosh Hashana falling out on Thursday (hei), both months of Cheshvan and Kislev being shalem (shin - 30 day months instead of possibly 29; as mentioned previously, these are the only months that can switch off in our set calendar),[13] and Pesach falling out on Sunday (aleph). As noted, this rare year type occurs on average only once in about 30.19 years (approximately 3.3 percent of the time),[14] and at times there are 71 years (!) in between HaSh”A years.[15]
Skip the Birthday
This background dramatically affects Bar Mitzvah bachurim. You see, if a boy was born on the 30th of Cheshvan - which is also the first day of Rosh Chodesh Kislev, in a year that Cheshvan was malei, and in his Bar Mitzvah year Cheshvan is a chaseir, meaning there is no 30th of the month, our little lad does not become a man until the 1st of Kislev.[16] In other words, he does not truly have a birthday, as in that year, his birthdate simply does not exist. Hence, his birthday is skipped and is pushed off one day until the next month. He may have been born in Cheshvan, but his Bar Mitzvah will assuredly be observed in Kislev.
The reason for this is is that halachically one cannot become truly a Bar Mitzvah until he completes 13 entire years.[17] Since there is no 30th of Cheshvan in his Bar Mitzvah year, he does not actually reach that milestone until the next day, which is the one-day Rosh Chodesh Kislev, a.ka. Alef Kislev.[18]
Bar Mitzvah-ed a Day Early
Yet, it is when the flip side of this equation occurs when it gets real interesting. The Elyah Rabba, based on a ruling of the Bach,[19] maintains that if a boy is born on the 1st of Kislev in a year when Cheshvan only had 29 days, and in his Bar Mitzvah year Cheshvan has 30 days, then the boy becomes Bar Mitzvah on the first day of Rosh Chodesh Kislev, which is actually the 30th of Cheshvan! Since he completes 13 full years on that day (as there now is an extra day added to that year), he is obligated in Mitzvos on the day prior to his birthday. His birthday might be Alef Kislev, but his Bar Mitzvah is Lamed Cheshvan.[20]
Although the Birur Halacha opines that it might still be preferable not to count this boy for a minyan until his actual birthday,[21] nevertheless, this fascinating, albeit relatively obscure psak of becoming a Bar Mitzvah one day before the birthday, is actually cited as halacha by the consensus of many great Acharonim, including the Shaarei Teshuva, Derech Hachaim, Rav Akiva Eiger, Pri Megadim, Machatzis Hashekel, Levushei Srad, Mishnah Berurah, and Kaf Hachaim.[22] In fact, Rav Shmuel Halevi Wosner explicitly ruled that “lema’aseh hachi nahug.”
This is why the great Bobover Rebbe zt”l claimed he became Bar Mitzvah one day before his Bar Mitzvah date. He was born on Alef Kislev in a year when Cheshvan was chaseir (5668), and in his Bar Mitzvah year (5681) Cheshvan was malei.
The same fascinating occurrence happens this year. Thirteen years ago, in 5772, Rosh Chodesh Kislev was a one-day Rosh Chodesh, as Cheshvan that year was chaseir. Yet, this year, 5785, as mentioned previously, both Cheshvan and Kislev are malei. Ergo, Rosh Chodesh Kislev is a two-day Rosh Chodesh. That means that a boy who was born on 1 Kislev 13 years ago (the singular one-day Rosh Chodesh Kislev that year), actually becomes Bar Mitzvah this year on Lamed Cheshvan, the first day of Rosh Chodesh Kislev, or, in other words, one day prior to his actual birthday! Fascinating, no? Just another noteworthy feature of our exceptional year.
No early Yahrtzeit
However, it is important to note that according to the halachic consensus, this rule does not actually apply regarding Yahrtzeits,[23] as a Yahrtzeit is strictly observed on the exact date when someone is niftar. Moreover, as Yahrtzeit observance is technically considered a neder (vow), the rule of thumb is that regarding nedarim, its observance follows the common vernacular. As Lashon Bnei Adam is to refer to the 1st of Kislev as Rosh Chodesh Kislev, that is the date that must be observed as proper Yahrtzeit, regardless of the month’s makeup.
Hence, in our current year, a Yahrtzeit for someone who was niftar 13 years ago on 1 Kislev (which, back in 5772 was a one-day Rosh Chodesh Kislev), would still be observed this year on 1 Kislev, even though this year it actually ends up being the second day of Rosh Chodesh Kislev.
This maxim is in contrast to Bar Mitzvahs, as a child becomes Bar Mitzvah on the day he completes 13 full years. In other words, and as diverging from Yahrtzeit observance, the upshot of this discussion is that a Bar Mitzvah technically does not necessarily have to occur exactly on the boy’s actual birthday, as is showcased with a Rosh Chodesh Kislev Bar Mitzvah on this rare year.
In conclusion, we find that the Gemara stresses the importance of knowing the calculations of our calendar,[24] with many Rishonim understanding that there is a specific mitzvah to do so.[25] The Chazon Ish explicitly mentions the calculations of the Lunar month as a prime example of this.[26] Our rare year gives us a plethora of opportunities to focus on as well. Now, at least we can all say that we are one step closer.
The author wishes to thank R’ Shloime Lerner,whose Mareh Mekomos served as the impetus for this author’s interest and research in this topic.
For any questions, comments or for the full Mareh Mekomos / sources, please email the author: [email protected].
Rabbi Yehuda Spitz, author ofM’Shulchan Yehuda and Peninim M’Shulchan Yehuda on Inyanei Halacha, serves as the Sho’el U’Meishiv and Rosh Chabura of the Ohr Lagolah Halacha Kollel at Yeshivas Ohr Somayach in Yerushalayim
Rabbi Spitz’s recent English halacha sefer,
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has more than 500 pages and features over 30 comprehensive chapters discussing a myriad of halachic issues relating to food, in an engaging manner. It is now available online and in bookstores everywhere.
[2]Rabban Gamliel’s statement (‘mekublani m’bais avi abba’) in Gemara Rosh Hashana (25a). See Rambam (Hilchos Kiddush HaChodesh Ch. 6:2 and 3), the Tur’s Seder HaKvius (Orach Chaim 427-428), the Mishnah Berurah’s Seder Laasos Luach B’Ketzara (end Orach Chaim 428), and Chazon Ish (Orach Chaim 138:4). This comes out to 29.53059 days. According to ‘Discovery’ by Rabbi Shraga Simmons (pg. 31), and Hamehapeich (by Rav Zamir Cohen; vol. 4, pg. 16-17), this data has recently been corroborated by NASA: “After years of research based on calculations using satellites, hairline telescopes, laser beams, and super computers, scientists at NASA have determined that the length of the ‘synodic month’, the time between one new moon and the next, is 29.530588 days.”
[3]See Ra’ah (Beitzah 4b) and Sefer Hachinuch (Parashas Bo, Mitzvah 4 – Kiddush Hachodesh).
[4]This is in a regular year; in a leap year Adar Rishon would have 30 Days and Adar Sheini would have 29.
[5]See Biur Halacha (428:1, end s.v. eilu hayamim).
[6]This wording follows the Tur and Shulchan Aruch’s opinion (Orach Chaim 427:1); this is also how the Aruch Hashulchan (ad loc.) concludes lemaaseh. However, the Bach (ad loc. s.v. v’kosvin) and Pri Megadim (ad loc. M.Z. 1) maintain that the order should be reversed and written as “Yom Shloshim L’Chodesh Av, Shehu Yom Rosh Chodesh Elul.” See Mishnah Berurah (ad loc. 1).
[7]Biur Halacha (428:1, end s.v. eilu hayamim). He also writes a tad earlier that “v’hinei kol zeh shekasavnu ain tzarich leha’arich b’frotrot aich hu kein, rak sheteida haklal,” (loosely) that all of these matters do not need to be measured in their exact minutiae, but rather one should know the general rules.
[8]Growing up in Miami Beach, one of the Bobover Rebbe’s prime Chassidim, Reb Yechiel Kurtz a”h maintained a vacation home directly across the street from us. He graciously allowed the Rebbe full use of his home, and the Rebbe zy”a would visit several times a year, often inviting us for Seudas Shlishis or Melaveh Malka. Years later, when visiting the Rebbe in New York, he would always greet us with a warm smile and referring to each of us as “Mein Alte Shachein.”
[9]See Gilyon Shoneh Halachos (Bobov; Chodesh Nissan 5771, pg. 11), in a letter from Rav Yehoshua Shalom Nosson Rubin. Thanks are due to R’ Shloime Lerner for providing this source.
[10]Rema (Orach Chaim 55:10; based on Shu”t Mahari Mintz 15), Levush (Orach Chaim 685:1), Magen Avraham (Orach Chaim 55:10), Pri Chodosh (ad loc. 10), Pri Megadim (ad loc. Eshel Avraham 10; he adds that m’pashtus this is also the Shulchan Aruch’s shittah), Levushei Srad (ad loc. s.v. eino), Korban Ha’eidah (on Yerushalmi Megillah Ch. 1, Shiyarei Hakorban s.v. hada), Shaarei Teshuva (Orach Chaim 55:11), Gilyon Maharsha (Yoreh Deah 402, s.v. b’Adar), Chayei Adam (vol. 1, 66:1), Kaf Hachaim (Palaji; 13:2), Kitzur Shulchan Aruch (15:2), Aruch Hashulchan (Orach Chaim 55:14), Aruch LaNer (Shu”t Binyan Tzion 151; he maintains that even if one was born on 30th of Shvat, a.k.a. the first day of Rosh Chodesh Adar in a standard year, gets pushed off until the first day of Rosh Chodesh Adar Sheini, as both days of Rosh Chodesh have the status of ‘Rosh Chodesh Adar’), Mishnah Berurah (ad loc. 45), and Kaf Hachaim (ad loc. 59); not like the Maharash Halevi (or Maharash L’Bais Halevi; Shu”t Orach Chaim 16) who was of the opinion that in such a case, all observances should be celebrated in Adar Rishon.
[11]Although the Magen Avraham (ibid., based on his understanding of the Mahari Mintz’s position) maintains that even a boy born in an Adar Rishon’s Bar Mitzvah gets deferred to Adar Sheini, and the Olas Hatamid (ad loc.) and Chasam Sofer (Shu”t Orach Chaim 163: end 3; although in Siman 14, he seems to contradict this position and rule accordingly to the majority consensus) agree with him, nevertheless, the consensus of poskim is that one who is born in an Adar Rishon’s Bar Mitzvah is observed in Adar Rishon as well; if he was born in a standard Adar or Adar Sheini, his Bar Mitzvah would be observed in Adar Sheini. These poskim include the Shulchan Aruch (ibid.), Levush (Orach Chaim 685 1), Pri Chodosh (ad loc. 10; citing the Yerushami Megillah Ch. 1:5, that Adar Rishon is merely a ‘tosefes’), Shvus Yaakov (Shu”t vol. 1:9; who writes that the Magen Avraham misunderstood the Mahari Mintz), Elyah Rabba (Orach Chaim 55:9 and Elyah Zuta 5), Rav Dovid Oppenheim (cited in the Ba’er Heitiv ad loc. 11), Me’il Tzadaka (Shu”t 21), Shaarei Teshuvah (Orach Chaim 55:11), Maamar Mordechai (ad loc. 13), Pri Megadim (ad loc. Eshel Avrohom 10), Ikrei Hadat (3:7), Chida (Machzik Bracha ad loc. 4), Machatzis Hashekel (ad loc. 10; he also questions the Magen Avraham’s understanding), Chayei Adam (vol. 1, 66:1), Kaf Hachaim (Palaji; 13:2), Maharsham (Daas Torah ad loc. s.v. u’shnas), Kitzur Shulchan Aruch (15:2), Aruch Hashulchan (Orach Chaim 55:14), Mishnah Berurah (ad loc. 43), and Kaf Hachaim (ad loc. 59). The Ba’er Heitiv (ibid.) concludes that “v’chein haminhag pashut eitzel kol ba’alei hahora’ah.” See also the Beis Yitzchok’s haskama to Rav Nosson Nota Landau’s Oorah Shachar, who as a side point, cites his ancestor the Maharam Padua as ruling this way as well. On an anecdotal level, this halacha affected this author growing up, as my birthday was Rosh Chodesh Adar and my Bar Mitzvah occurred on a leap year. This also affected my son who was born on my birthday as well, as his Bar Mitzvah was observed on Rosh Chodesh Adar Sheini 5776. [However, as an interesting counter-point, it had the opposite effect on his twin sister - as they were both born on Rosh Chodesh Adar Sheini and her Bas Mitzvah was on a standard Adar year. That means she reached the age of Mitzvos 13 months before her twin brother!]
[12]This distinction is detailed later in the article. See also footnote 23. This was discussed at length in a previous article titled “A Tale of Two Adars: Computations and Calculations – Bar Mitzvahs and Yahrtzeits.”
[13]According to R’ Yosef Yehuda Weber, author of Understanding the Jewish Calendar, Cheshvan and Kislev both having 30 days can only happen when the year has either 355 days or 385 days, which occurs pretty often - 44.93119 percent of the time.
[14]As noted by R’ Yosef Yehuda Weber, HaSh”A years occur only 8 times in the Tur’s entire 247 year cycle - a mere 3.23877 percent of the time. In fact, between the year 4119 (the year that the calendar was established) and year 6000, it will have occurred only 62 times – 3.29437 percent!
[15]Indeed, the last time this year type occurred was 31 years ago in 5754 / 1994. The next time will be 20 years hence in 5805 / 2044. The next several times after that are slated to be 27 years further, in 5832 / 2071 and then a 51 year gap in 5883 / 2122.
[16]Magen Avraham (Orach Chaim 55: end 10 s.v. u’mi), Elyah Rabba (ad loc. 9), Levushei Srad (ad loc. s.v. d’naaseh), Derech Hachaim (Dinei Kaddish B’Asarah 4), Shulchan Aruch Harav (Orach Chaim 55:13), Shaarei Teshuva (ad loc. 13), Mishnah Berurah (ad loc. 45), Aruch Hashulchan (ad loc. 15), and Kaf Hachaim (ad loc. 60).
[17]See Shu”t HaBach (Hayeshanos 145), SM”A (Even Ha’ezer 35:2), Magen Avraham (Orach Chaim 55:10), Ba’er Heitiv (ad loc. 11), Elyah Zuta (ad loc. 4), and Mishnah Berurah (ad loc. 45).
[18]Thanks are due to R’ Eli Dov Stern for pointing out this phenomenon.
[19]Elyah Rabba (Orach Chaim 55: end 9) based on the Bach (ibid.), although he implies that to his understanding, the Magen Avraham (cited above, that even a boy born in Adar Rishon’s Bar Mitzvah is observed in Adar Sheini) will not necessarily agree with this. However, the Pri Megadim (ibid.) counters that in his opinion, the Magen Avraham would indeed agree to this ruling of the Bach et al.
[20]A potential additional reason for this ruling is that the Aruch LaNer (Shu”t Binyan Tzion 151) maintains that both days of Rosh Chodesh have a status of one day; meaning they are considered somewhat connected. [Although admittedly, he is referring to the opposite case – that one who was born on the 30th of Shvat (a.k.a. the first day of Rosh Chodesh Adar) in a regular year, if his Bar Mitzvah is in a leap year it should be observed a month later – on Rosh Chodesh Adar Sheini. Although this is not the halacha pesuka, and Rav Shmuel Salant and the Maharil Diskin ruled against it (as cited in Shu”t Shevet Halevi vol. 10:105), nonetheless, in such a situation Rav Wosner himself (Shu”t Shevet Halevi ibid.) and Rav Shlomo Zalman Auerbach (as cited in Ishei Yisrael Ch. 15, footnote 26) recommended being machmir and to wait until Rosh Chodesh Adar Sheini for Inyanei Deoraysa, i.e. for him to Daven for the Amud, be Motzi others’ obligations, or get an Aliyah other than Maftir etc. Of course he should still wear Tefillin etc. from his true (mainstream) Bar Mitzvah date]. According to this shittah, once our growing lad’s 13 years are complete and it is already Rosh Chodesh, he would already be considered (at least somewhat of) a Bar Mitzvah, even though his true birthday is the following day (of Rosh Chodesh). The Aruch Hashulchan (Orach Chaim 55: end 15) implies this way as well, and this logic is also cited lemaaseh by the Vayaged Yaakov (Shemos, Mitzvosecha Sha’ashu’ai, Mitzvah 4) and his son the Vayaan Yosef (Greenwald; Shu”t vol. 1:33, 1 s.v. v’heishiv), and grandson, the Mishpatecha L’Yaakov (Shu”t 13), and is mentioned in Ishei Yisroel (pg. 135, Ch. 15, footnote 26, in the parenthesis).
[21]Birur Halacha (Rav Yechiel Avraham Zilber; in his comments on the Mishnah Berurah 55:45, vol. 1, pp. 131-133). He posits that although our protagonist may take on the Mitzvos of a Bar Mitzvah bachur, it is nonetheless preferable to wait until his actual birthday to count him for a minyan or zimun, unless it is shaas hadchak. This author finds it interesting that his proofs are based on the dinim of yahrtzeit, which technically are not truly the same. As previously mentioned, and although the Piskei Teshuvos (vol. 1, ad loc. pg. 492:20), seemingly quite oddly opines to follow the Birur Halacha and not the hachraas poskei hadoros, nonetheless, as proven, this is not the normative halacha.
[22]Shaarei Teshuva (Orach Chaim 55:11), Derech Hachaim (Dinei Kaddish B’Asarah, end 4), Rav Akiva Eiger (Orach Chaim 55:10 s.v. v’am”sh), Pri Megadim (ad loc. Eishel Avraham 10), Machatzis Hashekel (ad loc. s.v. ad sheyavru), Levushei Srad (ad loc. s.v. ad sheyavru), Mishnah Berurah (ad loc. end 45), Kaf Hachaim (Palaji 13:2 and Sofer - Orach Chaim 55:60), Amudei Hashulchan (on Kitzur Shulchan Aruch 15:1), and Ishei Yisrael (pg. 135, Ch. 15, footnote 26, in the parenthesis). The Pri Megadim adds that the Magen Avraham would even be maskim to this. The Aruch Hashulchan also (ad loc. end 15) implies this way. The Vayaged Yaakov (ibid.) and his son, the Vayaan Yosef (ibid.) rule this way as well. Rav Wosner’s Teshuva ruling this way is printed in Kovetz Kol Torah (vol. 53, Mador Halacha, pg. 183–184).
[23]An interesting side point is that according to the halachic consensus, this Bar Mitzvah ruling would not apply to Yahrtzeits, which, in such a case, the majority of poskim rule should be observed on the second day of Rosh Chodesh. See Yoreh Deah (402:1), Magen Avraham (Orach Chaim 568:20), Taz (Yoreh Deah 65:8), Chochmas Adam (171:11), Matteh Efraim (Dinei Kaddish Yasom, Shaar 3:7), Elef L’Matteh (ad loc. 8), Kitzur Shulchan Aruch (221:4), Aruch Hashulchan (Orach Chaim 568:15), and Gesher Hachaim (pg. 345); not like the Shu”t Panim Meiros (vol. 2:83; cited in Shaarei Teshuva 568:17). This distinction was discussed at length in a previous article titled “A Tale of Two Adars: Computations and Calculations – Bar Mitzvahs and Yahrtzeits.”
[24]Shabbos (75a).
[25]They are divided though, whether it is a Mitzvah Deoraysa or Derabbanan. The BeHa”G (Mitzvos Kum Asei 75), SMa”G (Asein 47), SMa”K (Mitzvah 103; however, he holds that this is part of the Mitzvah of Kiddush Hachodesh) and the Sefer Yereim (Mitzvah 60), count this as a Mitzvah Deoraysa, while the Rambam in his Sefer HaMitzvos (Shoresh HaRishon) and the Ramban (Hasagos ad loc.) write explicitly that they are are of the opinion that it is Derabbanan.
[26]The Chazon Ish (Orach Chaim 138:4) writes that knowing the calculations for the lunar month and other Tekufos is considered “Torah.” See also Rav Chaim Kanievsky’s introduction to his Shekel HaKodesh, where he recounts the importance and reverence the Chazon Ish held of calculating the months, and how he even would consider his mathematical calculations as “Geniza.” See Rav Yisroel Reisman’s excellent Pathways of the Prophets (pp. 272–311), who expounds upon and elucidates practical examples of understanding calendar calculations at length.
Disclaimer: This is not a comprehensive guide, rather a brief summary to raise awareness of the issues. In any real case one should ask a competent Halachic authority.
L'iluy Nishmas the Rosh HaYeshiva - Rav Chonoh Menachem Mendel ben R' Yechezkel Shraga, Rav Yaakov Yeshaya ben R' Boruch Yehuda.
