High Holidays · Sukkot and Simchat Torah
When is Kiddush Levanah said after Yom Kippur?
Kiddush Levanah, קידוש לבנה (the monthly blessing over the waxing moon), is customarily said after Yom Kippur — when we step out of the fast with a firm and joyful spirit and bless the renewing moon under the open sky.
Why after Yom Kippur
The blessing may be said from about three days after the molad (the moment the new moon is born) until roughly the middle of the month — but it needs one thing that no calendar can promise: a moon you can actually see. Since Yom Kippur falls just as this window is opening, the custom arose to say Kiddush Levanah on leaving the fast. There is a quiet beauty in it: we have just been forgiven and set right, and now, steady on our feet, we go outside and bless the moon that begins again — a mirror of our own renewal.
Where this comes from
The blessing over the new moon is discussed in the Talmud, Sanhedrin 42a, where greeting the moon's renewal is spoken of as a way of greeting the Presence of Heaven in the renewal of creation. The custom to say it in a settled, upright frame of mind — and so after Yom Kippur, rather than in the anxious hours before it — flows from that same seriousness: this is an encounter, not a routine.
What to do
After Yom Kippur, once the fast has ended and the moon is visible, go outside — the blessing is said standing, under the open sky, not through a window. Say it with joy; in many communities people greet one another afterward with shalom aleichem. If clouds hide the moon that night, wait: the window lasts until about mid-month, so you can say it on a later clear evening.
Source: Talmud, Sanhedrin 42a.
Because the whole thing depends on when the moon is actually visible over your city, let Barkai (the blessings of the day; the sky and place tell you the moment) tell you the night it is ready.
Open Barkai