When is the Blessing Administered?

In the Holy Temple, the Kohanim ascended a platform ("duchan"—thus the origin of the Yiddish term for the Priestly Blessing: "duchening") after the morning sacrificial offerings, and blessed the gathered throngs. With the destruction of the Temple, the blessing is administered in the course of the prayer services, during the chazzan's Repetition of the Amidah. It is necessary for a minyan to be present in order for the Kohanim to administer the Birkat Kohanim.

In Jerusalem, the Birkat Kohanim rite is performed every morning. On days when the Musaf service is recited, the Birkat Kohanim is performed both during Shacharit and Musaf. In all other Israeli cities beside Jerusalem, some (mostly Sephardim) perform Birkat Kohanim every day, while others (mostly Ashkenazim) only on Shabbat.

Birkat Kohanim is a throwback to the priestly Temple service. A Kohen who had imbibed even a small quantity of an intoxicating beverage was barred from performing any Temple service until the drink's effects had worn off. The same rule applies to Birkat Kohanim nowadays, and consequently the blessing is not administered during the afternoon Minchah service, for fear that some Kohanim may have enjoyed an aperitif together with their lunches. On public fast days the Kohanim recite the blessing during Minchah as well.

In the Diaspora

A blessing must be conferred with a joyful heart, hence the prevailing custom in the Diaspora to relegate the Birkat Kohanim to the major holidays. Furthermore, the blessing is only done during the Musaf prayer, when the crowd is happily anticipating their impending "dismissal" from synagogue, when they will be free to go home and celebrate the holiday meal with family and friends. Apparently, true joy cannot be experienced by all until the rabbi's sermon is dispensed with… An exception to this rule is Simchat Torah, when the Birkat Kohanim is done during the Shacharit (morning) services, this because on this joyous day many make kiddush (on alcoholic beverages) before Musaf.

Interestingly, the Birkat Kohanim is also performed on Yom Kippur; when we are joyful because of the atonement granted by G‑d on this holiest of days.

There are conflicting customs whether Birkat Kohanim is administered on a holiday which falls on Shabbat. Chabad custom is to proceed with the blessing as usual.