The Blessing over the Moon, also known as Kiddush Levana, “the Sanctification of the Moon,” is a blessing recited in the first half of a lunar month, while the moon is still waxing. The blessing is said outdoors, under the open sky, while one can see the moon in the sky. In this blessing, one thanks God for the cycle of the moon and the renewal of its light every month.

The source of the Blessing over the Moon is from the Babylonian Talmud, which also explains the importance of this blessing:

“When one blesses the new month in its proper time, it is as though he is greeting the Divine Presence… The school of Rabbi Yishmael taught: If the Jewish people merited to greet their Father in Heaven only one time each month, it would suffice for them. Abaye said: Therefore, we will recite the blessing while standing [in honor of the Divine Presence]” (Sanhedrin 42a).

The blessing combines a text from the Talmud (Sanhedrin 42a) with the addition of various psalms and prayers. At the end of Kiddush Levana, the Aleinu prayer is recited, even though it was already recited at the end of the evening service.

The blessing of the moon deals with the renewal of the moon, and therefore it is recited in the first half of each lunar month, while the moon is still growing fuller after its “birth.” According to the Ashkenazic custom, the blessing is said after at least three days have elapsed since the time of the molad. Sephardim and Hasidim wait for seven days from the molad before reciting it. These dates are usually indicated in the calendars that list halakhic times.

The most recommended time for Kiddush Levana is on a Saturday night after the evening service. At this point there is a relatively large congregation in attendance, which lends honor to the occasion. Furthermore, on Saturday night everyone is still wearing their more formal Shabbat attire, which is also a mark of respect for the ritual.

If one did not perform Kiddush Levana on Saturday night, he may do so on any day of the week, within the dates specified above.

Kiddush Levana is performed when it is dark outside, and when the moon is shining brightly and is not hidden by clouds. If clouds are covering the moon to the extent that it cannot be seen clearly, even if one can make out the halo of its light, one should not recite Kiddush Levana. If the clouds are not obscuring the moon entirely, and its form can be discerned, there are some (primarily Ashkenazim) who recite the blessing in such a situation, whereas others (especially Sephardim) do not do so.