During the time of the Temple, the Kohanim would recite the Priestly Blessing after the conclusion of the Korbanot service. The Torah writes this in reference to the eighth day of the dedication of the Tabernacle, the day the Kohanim began performing their service there. “Aharon lifted his hands toward the people and blessed them. He then descended from [the altar where he] had performed the sin offering, the burnt offering, and the peace offering” (Leviticus 9:22). From here we learn that the people were blessed at the conclusion of the sacrificial offerings. The reason for this is that after offering the sacrifices, which express our readiness to surrender ourselves and sacrifice our souls to Hashem, we are worthy of receiving His blessing.
Outside the Temple, the Chachamim established that Birkat Kohanim would be recited during the prayer service, since prayer replaced the Korbanot. There is a similarity between the Korbanot and prayer, for in both, one’s yearning for Divine closeness is expressed. Just like the Kohanim recited the blessing after the Korbanot, so it was established that towards the end of the Shemoneh Esrei the Kohanim bless the nation.
In order to greatly emphasize the relevance of Birkat Kohanim to the conclusion of the sacrificial offerings, the Chachamim established that the Kohanim are obligated to start walking in the direction of the duchan at the time of Birkat Retzeh, for that is the berachah that discusses the return of the sacrificial offerings. Any Kohen who does not head toward the duchan (“lift his feet”) at the time of Birkat Retzeh loses the opportunity to fulfill the mitzvah and is prohibited from ascending to the duchan during that Amidah (Shulchan Aruch 128:8). L’chatchilah, the Kohen should lift his feet in the beginning of Birkat Retzeh. However, as long as the chazan did not reach the end of the berachah, the Kohen is still permitted to ascend towards the duchan (Mishnah Berurah 128:25).
A Kohen who arrives late and realizes that he will not complete the washing of his hands before the conclusion of Birkat Retzeh is advised to take a small step in the direction of the duchan while still standing at the sink. With this step he is considered to have started to walk towards the duchan during Birkat Retzeh. When he finishes his washing, he continues to walk to the duchan to bless the people (see Mishnah Berurah 128:27, 28; Sha’ar HaTziyun 30; Sefer Nesiat Kapayim Kehilchatah 7:2, note 8).
Because Birkat Kohanim is connected to the Korbanot, the Chachamim teach that just as the time to bring sacrificial offerings was specifically during the day, so too, Birkat Kohanim is recited during the day. Therefore, the Kohanim do not bless the nation during Ma’ariv. Although presumably there is reason to recite Birkat Kohanim during Minchah, the Chachamim decided against this since the time of the Minchah prayer is usually after a meal, and there is concern that a Kohen who drank wine at his meal will subsequently ascend the duchan to bless the people and desecrate the berachah. It is forbidden for a Kohen who is drunk to work in the Temple, and he is prohibited from blessing the people (Shulchan Aruch 128:38). However, in Ne’ilah, and during Minchah of public fast days, the blessing is recited. Since the Kohanim are fasting, there is no concern of intoxication (Shulchan Aruch 129:1). This is on condition that the Minchah of a fast day is prayed after plag haminchah (a proportional hour and a quarter before sunset), but when Minchah of a fast day is recited before plag haminchah, the Kohanim do not bless the people.
. Although some maintain that it is permissible to bless the nation even at Minchah Gedolah of a fast day, the majority of poskim maintain that the blessing is only recited after plag haminchah. This is because the enactment to bless the nation in the afternoon pertains to the Ne’ilah prayer and not the Minchah prayer (on Yom Kippur, the Kohanim do not bless the people at Minchah). If Birkat Kohanim in the afternoon would have been instituted for the Minchah prayer, people might think that Birkat Kohanim is recited at Minchah of a regular day as well, when it is in fact forbidden for concern of intoxication. However, on a fast day in which there is no Ne’ilah, the blessing is recited at Minchah, on condition that it is recited at the time of Ne’ilah, meaning close to the end of the day.
Even though there are various opinions as to how to calculate plag haminchah (a proportional hour and a quarter before sunset, or a proportional hour and a quarter before tzeit hakochavim), the main calculation is a proportional hour and a quarter before sunset, since the opinion maintaining that it is before tzeit hakochavim is calculated based on Rabbeinu Tam (meaning, 72 minutes after sunset, as explained earlier in this book, chapter 11, note 14. In other words, plag haminchah according to Rabbeinu Tam is between two to eighteen minutes before the visible sunset.) Nonetheless, it is best to start Minchah a little later than that, approximately half an hour to three quarters of an hour before sunset, closer to the time of Ne’ilah. This way, Birkat Kohanim will be recited as close as possible to sunset.