Birkat HaLevanah: Sephardi and Ashkenazi Practice

Answer

Shalom u'vrachah,

The Beit Yosef (siman 426) cited in the name of Rabbeinu Yonah that one should not recite the brachah over the moon until it has "sweetened," that is, from the time its light is pleasant and a person derives benefit from it, and this is three days after the molad of the moon. And the Beit Yosef cited in the name of R. Y. Gikatilla that according to the Kabbalah one should not recite the brachah over the renewal of the moon until seven days have passed over it.

The Shulchan Aruch (426:4) ruled like R. Y. Gikatilla, that one should not recite the brachah over the moon until 7 days have passed from the time of the molad.

And the Mishnah Berurah (letter 20) wrote that the Acharonim disagree and recite the brachah over the moon after 3 days from the time of the molad.

Ashkenazim follow the Mishnah Berurah, reciting the brachah after 3 days, and Sephardim follow the Shulchan Aruch, reciting the brachah after 7 days from the molad (Shu"t She'ilat Shlomo 1:199). And when clouds are present during most of the days of the month, and if they wait to recite the brachah until 7 days have passed from the molad there is a concern of losing Birkat HaLevanah that month, then even for Sephardim one should recite the brachah after 3 days (see Yalkut Yosef 426:26).

In general, it is preferable to recite Birkat HaLevanah on Motza'ei Shabbat (Shulchan Aruch 426:2), and one should examine in the poskim the implications of this for the proper time for Birkat HaLevanah.