• Concept of bereira determining the halachic status of something in the present on the basis of what will develop in the future
• Applying this concept to eiruv, tithing and redemption of Second Tithe crops
• Making an eiruv when Shabbat and Holiday are back to back or for two days of Rosh Hashana
• Why Rabbi Sheishet refused to eat the deer meat
• Is there a mention of Rosh Chodesh in the Rosh Hashana prayers?
• The blessing of shehechiyanu on Rosh Hashana and Yom Kippur
• The clash between ending a fast and starting the Shabbat
• Leaving the techum the limit to which one may walk beyond his city on Shabbat and returning there
• Those who are victims of circumstances which cause them to lose control of their actions
• Beginning the Shabbat in an open valley which becomes fenced in that day
• The Sages on the ship on Shabbat
• Is flying by Eliyahu limited by the techum of Shabbat and why wont he arrive on Friday
• Rabbi Gamliels telescope

The general rule is that if any of the days in which Jews are obligated to fast in mourning for the destruction of the Beit Hamikdash falls on Shabbat, the fast is postponed to the following day so as not to interfere with the celebration of Shabbat.

One of the early commentators, Rabbi David Avudraham, is quoted by Rabbi Yosef Karo in his commentary on Tur Orach Chaim (550), as stating that the Fast of the Tenth of Tevet is an exception to rule. This fast day, which recalls the beginning of the Babylonian armys siege of Jerusalem which culminated in the destruction of the first Beit Hamikdash, can never fall on Shabbat according to our perpetual calendar. But if it could, as was possible in earlier times when the calendar was determined by the sighting of the new moon, we would be obligated to fast even though it was Shabbat. His logic is that in the prophecy of Yechezkel regarding the beginning of the siege, the term "on this very day" used to describe it is the same as that used in the Torah to describe Yom Kippur, and that just as we fast on Yom Kippur when it falls on Shabbat the same applies to the Tenth of Tevet.

Although no Talmudic source is mentioned for this unconventional position, it is suggested by the author of "Ohr Somayach" on the Mishneh Torah of Rambam that the source may be found in our gemara. The Sage Rabbah asked Rabbi Huna whether an individual yeshiva student who committed himself to fasting on Friday must complete his fast even though he enters Shabbat in a suffering state. Why, asked the "Ohr Somayach", didnt he pose the same question in regard to the four mandatory communal fasts? The answer is that in his time the perpetual calendar was already in effect and the only one of the fast days which could fall on Friday is the Tenth of Tevet. There could be no question regarding extending that fast day into Shabbat because that particular fast day would be observed on Shabbat itself if the two collided. Based on this analysis, he concludes, Rabbi David Avudraham arrived at his position.

"If one failed to include the blessing of shehechiyanu in the kiddush for a holiday, he must make it whenever he remembers, even if he is out on the street."

• Rabbi Nachman - Eiruvin 40b