To bless the new moon at the proper time is like greeting the Divine Presence.Talmud

Once a month, as the soft, mellow light of the moon waxes in the sky, we recite a special blessing called Kiddush Levanah.

Kiddush Levanah can be recited three days after the moon’s rebirth—known as the molad. Kabbalah tells us it is best to wait a full week. Once fifteen days have passed, the moon begins to wane once more, and the season for saying the blessing has passed. The very best time for this blessing is immediately after Shabbat, as we leave the synagogue together dressed in our Shabbat clothes.

We’re not praising the moon, but its Creator—for His wondrous work we call astronomyHere’s the formula: After nightfall, when the moon is fully visible and unobstructed by cloud cover, stand under the open sky and look once at the moon. Then face east and recite the blessing, along with a few accompanying prayers. The more people you get to participate, the better—you are, after all, greeting the divine presence. If you have a quorum of ten men, kaddish is recited.

Of course, we’re not praising the moon, but its Creator—for His wondrous work we call astronomy. The moon has the most obvious monthly cycle of all the stars and planets, so we take the occasion of its renewal to make a blessing for the entire masterpiece.

Besides, our nation is likened to the moon—as it waxes and wanes, so have we throughout history. So, too, has the divine presence (the Shechinah)—which explains why the moon represents that, as well. So blessing the moon on its reappearance is a way of renewing our trust that the light of G‑d’s presence will soon fill all the earth, and our people will be redeemed from exile, very soon in our time. Which is why the blessing is concluded with songs and dancing in celebration and joy.