Installation. Some indication of the office Jehovah had in mind for Aaron is seen in privileges given him soon after the Exodus from Egypt. In the wilderness on the way to Sinai, Aaron was the one commanded to take a jar of manna and to deposit it before the Testimony as something to be kept. This was before the tent of meeting or the ark of the covenant was yet in existence. (Ex 16:33, 34, ftn) Later, Aaron came to be the one in full charge of the sacred tent and its Ark. Aaron and two of his sons, with 70 of the older men of Israel, were specifically named as privileged to go partway up Mount Horeb, where they saw a vision of God.​—Ex 24:1-11.

But Jehovah made his first actual statement of his purpose to separate Aaron and his sons for the priesthood when giving Moses instructions for making the priestly garments. (Ex 28) After these instructions were given, God outlined to Moses the procedure for installing the priesthood and then definitely made it known: “The priesthood must become theirs as a statute to time indefinite.”​—Ex 29:9.

In keeping with Jehovah’s majesty and cleanness, Aaron and his sons could not perform priestly duties until they were sanctified and empowered by the installation service. (Ex 29) Moses, as mediator of the Law covenant, performed the installation. A sanctification ceremony, occupying the seven days of Nisan 1 to 7, 1512 B.C.E., saw the priesthood fully installed, their hands filled with power to act as priests. (Le 8) The next day, Nisan 8, an initial atonement service was performed for the nation (very much like the regular Day of Atonement services that were decreed to be celebrated annually on Tishri 10; this first performance of the priesthood is described in Leviticus 9). It was appropriate and necessary, for the people of Israel were in need of cleansing from their sins, including their recent transgression in connection with the golden calf.​—Ex 32.

In installing the high priest, one of the significant acts Moses had to perform was the anointing of Aaron by pouring upon Aaron’s head the sacred anointing oil specially compounded according to God’s directions. (Le 8:1, 2, 12; Ex 30:22-25, 30-33; Ps 133:2)

The appointment of Aaron, Israel’s first high priest, was from God. (Heb 5:4) The high priesthood of Israel was inaugurated in Aaron and passed down from father to oldest son, unless that son died or was disqualified, as in the case of Aaron’s two oldest sons, who sinned against Jehovah and died. (Le 10:1, 2)

Aaron was 83 years old when he went with Moses before Pharaoh. His anointing as high priest apparently took place in the following year. (Ex 7:7) He was 123 years of age at the time of his death. During all this time he served, with no retirement. (Nu 20:28; 33:39)