There is a mitzvah to say a special bracha at the beginning of each new month. This bracha is referred to in halacha and in the vernacular as birkat ha’levana or kiddush levana. It is said by the congregation outside the synagogue after they briefly look at the new moon, and is usually said immediately after the evening prayers that conclude the Shabbat. The text of the bracha and accompanying words of praise to Hashem are found in any siddur — and sometimes even on the outside front wall of the synagogue!

The commentaries explain the significance of the statement in our gemara, which connects the new moon and King David. Rashi, on our daf, cites a verse in Tehillim (89:37) that draws a comparison between the moon and the royal dynasty of King David: “[The Kingdom of David] is like the moon, which is established forever and is a witness in the sky.” Rashi in Tanach explains: “The moon and the sun are witnesses to him [King David] — as long as they exist, his kingdom will exist, as it is written (Yirmiyahu 33:20), ‘If you break My covenant with the day, and My covenant with the night… also My covenant with David will be broken.’”

The accepted practice nowadays to mention these words of about King David as part of kiddush levana is first codified by the Rema in Shulchan Aruch Orach Chaim 426:2. The Rema writes: “Our custom is to say ‘David, melech Yisrael, chai v’kayam’ (David, the King of Israel, is alive and well).’ Why? Since David’s kingdomis compared to the moon (in Tanach), his kingdom will therefore be renewed in the future, just as the moon is renewed nowadays monthly. When King David’s dynasty is renewed, the Jewish nation will then again cleave to its ‘Husband,’ meaning the Holy One, Blessed be He. This connection between the Jewish People and Hashem is similar to the manner in which the moon renews itself in connection with the sun. As a result of our renewed awareness of Hashem’s promise to restore the Davidic dynasty and draw us closer to Him, we celebrate at this time with dancing and other signs of joy, as we would do at a wedding.” (Based on Rabbeinu Bechayei to the Torah portion of Vayeishev; see also Maharitz Chuiyus, Maharsha and Midrash Rabbah Shemot 15.)

Other expressions of our happiness at the time of kiddush levana — due to our special relationship with Hashem and the restoration of King David’s dynasty with the arrival of the Mashiach (a descendant of King David) — are part of this monthly ceremony. For example, we say three times “Siman tov u’mazal tov” — “May this be a good sign and good fortune for us and the entire Jewish nation.” In addition, each person in the group says to three nearby people, “Shalom Aleichem,” thus wishing them peace, happiness and success.

Some sources write that the phrase “David, King of Israel, is alive and well” has the same gematria as “Rosh Chodesh.” Iyun Tefillah, a commentary on the siddur by Rabbi Yaakov Tzvi Mecklenburg, suggests that this gematria equivalence was indeed Rebbi’s intention when requesting those specific words as a sign of the declaration of a new month, a sign or renewal.