Many Rishonim maintain that the time for reciting Birkat Ha-levana begins on the first day the moon is visible, and that the earlier one recites the berakha the better (mt, Laws of Berakhot 10:17; Rosh; and others). Several poskim, however, posit that it is preferable to wait until the moon grows somewhat, when it is possible to benefit from its light. Some say that one should wait until three whole days pass, as that is when the moon’s light begins to be usable (R. Saadia Gaon, Talmidei Rabbeinu Yona), while others say that seven days must pass, because that is when one can begin to truly benefit from its light (Responsa Rema Mi-Fano §78). According to some of the greatest kabbalists – most notably, R. Yosef Gikatilla – one should wait seven days, for kabbalistic reasons. They explain that the renewal of the moon’s light alludes to man’s renewal, and whenever there is a new development there is concern that God’s attribute of justice might prosecute and hinder the new growth. Therefore, it is proper to wait seven days, like the seven days of creation, since by then the moon’s light will have stabilized and the attribute of justice can no longer prosecute against the new beginning.

The Sephardic and Hasidic custom is to wait until the seventh of the month to recite the berakha (sa 426:4), while the Ashkenazic custom is to recite it after three days have passed (Baĥ, mb 426:20). In practice, though, it is customary to recite Birkat Ha-levana on Motza’ei Shabbat, so that the berakha can be recited joyously, while wearing nice clothing. Thus, practically speaking, Ashkenazim and Moroccan Jews recite Birkat Ha-levana on the Motza’ei Shabbat that follows three whole days after the molad, while Sephardim and Ĥasidim recite it on the first Motza’ei Shabbat after the seventh of the month.

There is a dispute regarding how to act when the seventh of the month begins on a Motza’ei Shabbat, but seven complete days from the time of the molad (168 hours) have not yet passed. Some authorities maintain that we postpone the berakha until the next night or the next Motza’ei Shabbat, which will fall out on the fourteenth of the month (Rashash, R. Zalman of Liadi, Kaf Ha-ĥayim 426:61). Others contend that even if a few hours remain until the end of the seventh day after the molad, one may recite Birkat Ha-levana (Knesset Ha-gedola, Yeĥaveh Da’at 2:24). If people from different ethnic backgrounds pray together and the seventh of the month falls out on a Motza’ei Shabbat, it is proper for everyone to recite Birkat Ha-levana, as that is the position of the majority of poskim.[24]

One who failed to recite Birkat Ha-levana by the seventh of the month can recite it until the end of the night of the fifteenth, because the moon is still waxing until then. Afterward, however, the moon begins to wane; therefore, the berakha cannot be said from the night of the sixteenth and onward (sa 426:3).

Preferably, one should take into account the opinion of Maharil, who maintains that one may not recite Birkat Ha-levana after half of the lunar cycle has elapsed (fourteen days, eighteen hours, and approximately twenty minutes, from the time of the molad). This time rarely elapses by the beginning of the night of the fourteenth. By the night of the fifteenth, the lunar cycle sometimes reaches the halfway mark and sometimes does not (Rema 426:3, Kaf Ha-ĥayim 426:53). Nevertheless, in practice, one who did not recite the berakha by the night of the fourteenth may recite it until the end of the night of the fifteenth (bhl 426:3, Yabi’a Omer 8:42).