Mishneh Torah (Rambam) משנה תורה

In the morning, Musaf, and Ne'ilah services, the priests recite the priestly blessing. They do not recite the priestly blessing in the Minchah service, because, by the time of the Minchah service, all the people have eaten. The possibility exists that the priests would have drunken wine, and it is forbidden to recite the priestly blessing while intoxicated. Even on a fast day, the priestly blessings are not recited during the Minchah service. This decree [was instituted,] lest [people fail to differentiate between] the Minchah service of a fast day and the Minchah service of an ordinary day.

When does the above apply? On fast days when both Minchah and Ne'ilah are recited - i.e., Yom Kippur and communal fasts [declared because of distress]. However, on a fast day on which Ne'ilah is not recited - e.g., Tish'ah B'Av or the seventeenth of Tammuz - since the Minchah service is recited close to sunset, it resembles Ne'ilah and will not be confused with an ordinary Minchah service. Therefore, the priestly blessing is recited during it. If a priest transgressed and ascended to the platform during the Minchah service of Yom Kippur, since it is known that there is no possibility of drunkenness on that day, he may recite the priestly blessing, and he is not required to descend because of the suspicion [that might be aroused], so that people do not say, "He is of blemished lineage. Therefore, they forced him to descend."

How is the priestly blessing recited outside the Temple? When the leader of the congregation reaches the blessing R'tzey, when he recites the word R'tzey all the priests in the synagogue leave their places, proceed forward, and ascend the duchan. They stand there, facing the heichal, with their backs to the congregation. They hold their fingers closed, against their palms, until the leader of the congregation completes the blessing Modim. [Then,] they turn their faces to the people, spread out their fingers, lift up their hands shoulder high, and begin reciting, Y'varechecha.... The leader of the congregation reads [the blessing] to them, word for word, and they respond after him [as can be inferred from Numbers 6:23: "This is how you should bless the children of Israel:] 'Say to them...;’” [i.e., the priests do not bless until one] "says to them." When [the priests] conclude the first verse, all the people answer "Amen." The leader of the congregation reads [the priests] the second verse, word for word, and they respond after him until they complete the second verse. The people respond "Amen." The same applies regarding the third verse.

When the priests conclude the recitation of [these] three verses, the leader of the congregation begins the final blessing of the Shemoneh Esreh, Sim shalom. The priests turn their faces to the ark and close their fingers. They remain standing on the duchan until the leader of the congregation concludes the blessing, [and then] return to their places.

The person who calls the priests is not permitted to call to the priests until the Amen of the community is no longer heard. The priests are not permitted to begin reciting the blessing until the statement of the person who calls the priests is no longer heard. The congregation should not respond "Amen" until the blessing of the priests is no longer heard. The priests may not begin another blessing until the Amen of the community is no longer heard. The leader of the congregation is not allowed to recite Amen to the priests' blessings like the rest of the people, lest he become confused and not realize which blessing to recite to them, whether the second blessing or the third blessing.

The priests are not permitted to turn their faces away from the congregation until the leader of the congregation begins [the blessing] Sim shalom. Neither may the priests leave their places until the leader of the congregation concludes [the blessing] Sim shalom, nor may they close their fingers until they turn their faces from the community. One of the measures ordained by Ezra is that the priests should not ascend to the duchan wearing sandals. Rather, they should stand barefoot.

When the priests bless the people, they should not look at them or divert their attention. Rather, their eyes should be directed towards the earth like one standing in prayer. A person should not look at the priests' faces while they are blessing the people, lest they divert their attention. Rather, all the people should listen attentively to the blessing; they should [stand] face to face with the priests, without looking at their faces.

If only one priest is blessing the people, he should begin reciting the blessing alone. [Afterwards,] the leader of the congregation reads [the blessings] to him, word for word, as mentioned. If there are two or more [priests blessing the people], they do not begin reciting the blessing until the leader of the congregation calls them, saying "Kohanim." They answer and respond Y'varechecha, and then he reads [the blessings] to them, word for word, in the manner described above.

How is the priestly blessing recited in the Temple? The priests ascend to the duchan after the priests have completed the service associated with the morning sacrifice offered daily. They lift their hands above their heads with their fingers extended, except the High Priest. He does not lift his hands above the tzitz. One person reads [the blessings] to them, word for word, in the same manner as outside the Temple, until they complete the three verses. The people do not respond ["Amen"] after each verse. Instead, in the Temple, [the priestly blessings] are read as a single blessing. When [the priests] conclude, all the people respond, "Blessed be God, the Lord, the Lord of Israel to all eternity."

They recite [God's] name - i.e., the name י-ה-ו-ה , as it is written. This is what is referred to as the "explicit name" in all sources. In the country, it is read [using another one of God's names]: אדני, for only in the Temple is this name [of God] recited as it is written. After Shimon HaTzaddik died, the priests ceased reciting the [priestly] blessing using God's explicit name even in the Temple, lest it be learned by a person lacking proper stature and moral conduct. The Sages of the early generations would teach [this name] once in seven years, only to their students and sons [who had proven] their moral conduct. All this is in reverence for His great and awesome name.

Wherever the priestly blessing is recited, it is recited only in the holy tongue, as [implied by Numbers 6:23]: "This is how you should bless the children of Israel." We have learned the following [instructions] from the tradition [passed on] from Moses, our teacher, may he rest in peace: "This is how you should bless" - while standing. "This is how you should bless" - raising your hands. "This is how you should bless" - in the holy tongue. "This is how you should bless" - face to face. "This is how you should bless" - in a loud voice. "This is how you should bless" - mentioning [God's] explicit name; the latter [applying only] when one is in the Temple, as explained.

Wherever [they recite the blessing], the priests are not permitted to add other blessings - e.g., "May God, Lord of your fathers, increase your numbers a thousandfold" (Deuteronomy 11:11) - in addition to the three verses [of the priestly blessing]. [These additions may not be made] either silently or out loud, as [Deuteronomy 4:2] states: "Do not add to the matter." While a priest is ascending to the duchan, he recites [the following prayer] when he leaves his place to ascend: May it be Your will, God, our Lord and Lord of our fathers, that this blessing which You have commanded us to bless Your people, Israel, be a perfect blessing, that it not be marred by obstacles or iniquity, from now until eternity. Before he turns to bless the community, [a priest] should recite the blessing: Blessed are You, God, our Lord, King of the universe, who has sanctified us with the holiness of Aharon, and commanded us to bless His people Israel with love. Afterwards, he turns his face to the community and begins reciting the priestly blessings. When he turns his face from the community after completing [the recitation] of the blessings, he recites [the following]: We have carried out that which You have decreed upon us. Deal with us as You have promised us: Look down from Your abode, from the heavens, and bless Your people, Israel.

When the priests turn their faces to the community to bless them, and when they turn their faces from the community after blessing [them], they should turn only to the right. Similarly, any turns which a person makes should always be to the right.

In the Temple, the priestly blessing would be recited once a day, after the [offering of] the morning sacrifice. [The priests] come and stand on the steps to the Ulam and recite the blessing, as mentioned above. However, outside the Temple, it is recited after every prayer service, except Minchah, as explained. In all places, an effort is made that the person who reads the blessing to the priests should be an Israelite, as [implied by Numbers 6:23]: "Say to them." This implies that the one who reads the blessing to them is not one of them.