…You shall be inaugurated for a period of seven days.(Lev. 8:33)

The days of inauguration were seven, as the people already realized the significance of the seven garments of the High Priest [the trousers not being a specific garment of the High Priest]. We read in Isaiah (63:12): "…who made His glorious arm march at the right side of Moses." This is a reference to Moses, who trained Aaron and gave him some of his prophetic powers.

According to the Zohar, (parashat Tzav 34:2), this is why Moses had to anoint Aaron. During these seven days, Moses performed the High Priest's duties [dressed only in a white tunic] in order to confer all this sanctity upon Aaron.

[In the Zohar] Rabbi Abba said that since Moses was firmly rooted in the holy place whence all his power stemmed, he was able to transfer this sanctity to Aaron. This is why the Torah wrote earlier, "He poured from the oil of anointment on Aaron's head, and he anointed him and sanctified him." (Lev. 30:12)

This is also the reason that these days of inauguration were described [in the Zohar] as "days of perfection, or perfecting".

[Selected with permission from the seven-volume English edition of "The Torah Commentary of Rebbeinu Bachya" by Eliyahu Munk.]