BIRKAT KOHANIM - THE PRIESTLY BLESSING
We are all familiar with the beautiful blessing of the kohanim: "May HaShem bless you and watch over you..." This threefold blessing first appears in the Torah and it was uttered aloud each day by the kohanim in the Holy Temple. To this day kohanim, the descendants of Aharon, brother of Moshe, Israel's first Kohen Gadol, (High Priest), and the father of all kohanim ever since, continue to bless their brethren each day by uttering the exact same blessing.
SOURCE OF THE BLESSING
The blessing of the kohanim in the Holy Temple was part of the daily Tamid service. The source of the blessing is found in Numbers 6:22-27:
"HaShem spoke to Moshe saying: Speak to Aharon and his sons, saying: This is how you shall bless the children of Israel, saying to them: 'May HaShem bless you and watch over you. May HaShem cause His countenance to shine to you and favor you. May HaShem raise His countenance toward you and grant you peace.' They shall bestow My Name upon the children of Israel, so that I will bless them."
From this the sages learned some of the laws of the priestly blessing (birkat kohanim), such as: birkat kohanim must always be said everywhere in the sacred Hebrew language.
Furthermore, the blessing should be said while the kohanim are standing, with their hands raised and in a loud voice.
In addition, birkat kohanim is also mentioned in Leviticus 9:22:
"And Aharon lifted up his hands towards the people and blessed them. He then descended from preparing the sin offering, the burnt offering, and the peace offering."
From this the sages learned that the blessing must be said with the palms raised.
This was the day of the inauguration of the desert Tabernacle and it was the first time the blessing was uttered. Likewise, throughout the era of the Holy temple the blessing was said using the ineffable name of HaShem.
BIRKAT KOHANIM IN THE DAILY TAMID SERVICE
In the Holy Temple the kohanim performed the blessing during the morning Tamid service, following the placing of the organs of the Tamid lamb offering upon the altar fire.
The blessing was performed while the kohanim stood upon the steps leading up to the entrance to the Holy Temple.
NESIAT KAPAYIM - RAISING THE HANDS
The kohanim raised their hands above their heads and with outstretched fingers blessed the people.
The Kohen Gadol, (High Priest), however, would spread his hands keeping them below the tzitz - the golden crown which was placed upon his forehead and upon which the words "Holy to HaShem" were engraved.
The kohanim stood with their faces to the crowd and their backs to the Holy Temple, as the sages said: 'Thus bless the children of Israel, face to face you say face to face... as a man speaking to his friend."
From this the kohanim also learned how they must respect the public
"Rabbi Yitzchak says: The awe of the public should always be upon you, i.e., one must always treat the public courteously. As when the kohanim bless the people they face the people and their backs are toward the Divine Presence, out of respect for the congregation."
HALACHA: RULES & REGULATIONS CONCERNING BIRKAT KOHANIM
The halacha is that the kohanim must recite the blessing, each word by itself, and in the Temple the three verses are recited in their entirety without a break between each blessing. The people would not say 'Amen' between verses. (Today we say "amen" after each of the three blessings.)
Following the conclusion of the blessing of the kohanim , the people answered: "Blessed be G-d, the G-d of Israel, forever and ever." Some opine that they answered - "Blessed be the glory of His majesty forever and ever."
In the beginning, the kohanim would say in this blessing saying the ineffable name of HaShem. Following the death of Kohen Gadol Shimon Hatzadik (the Righteous), the kohanim stopped blessing using the ineffable name of G-d, so that they would not come to use the name inappropriately. Some say that after the uttering of G-d's ineffable name, the people prostrated themselves on the ground, just as they did when the ineffable name was uttered by the kohanim on Yom Kippur.
The sages extolled the importance of this mitzvah of blessing the people, saying: "Any kohen who does not go up to bless the people has transgressed on three commandments: [a] Thus shall you bless the children of Israel. [b] Say to them. [c] And put my name."
A kohen who does not bless - is himself not blessed. And every kohen who blesses - is blessed as it is said: "I will bless those that bless you." (Genesis 12:3)
THE SHULCHAN ARUCH: LAWS OF BIRKAT KOHANIM
The commandment for kohanim to bless the people each morning remains to this day, despite the destruction of the Holy Temple. Today the blessing is said by kohanim as part of the morning prayer each morning, and a second time on Shabbat and on holidays in which a Musaf prayer is said recalling the additional offerings performed on that day. This is the practice in Israel, and in some congregations throughout the diaspora. In many congregations throughout the diaspora, however, birkat kohanim is said only on the days of the three pilgrimage festivals, Passover, Shavuot and Sukkot, as well as Rosh HaShana and Yom Kippur.
The Shulchan Aruch, the universally accepted halachic guide, written in the 16th century by Rabbi Yosef Karo, describes how the blessing is to properly be performed:
Shulchan Arukh, Orach Chayim 128:5-6
Kohanim may not ascend to the platform in shoes, but in socks it is permitted. (Kohanim in the Holy Temple were barefoot.)
Even though the kohanim have washed their hands in the morning, they go back and wash their hands again until the wrist (that is, the joint connecting the hand and the arm). The Levi pours water on their hands, and prior to this, the Levi washes his [own] hands. Ram"a: We do not have the custom that the Levis wash their hands first; rather they rely on their morning's washing.
Shulchan Arukh, Orach Chayim 128:11
When they [the kohanim] turn their faces toward the people, they bless: "Blessed are You, HaShem our G-d, Sovereign of all, Who has sanctified us with the sanctity of Aharon and commanded us to bless His people Israel with love."
Shulchan Arukh, Orach Chayim 128:12
They raise their hands opposite their shoulders, and raise the right hand slightly above the left, and stretch out their hands and separate their fingers, and they aim to make five spaces: between two fingers and the other two fingers is the first space [on each hand]; between the index finger and the thumb; and from thumb to thumb. They spread their palms so that the interior of their palms faces the ground and the backs of their hands faces heaven.