During the week of Adar 23–29, 2449, Moses installed Aaron and his sons as priests. On the eighth day, the 1st of Nisan, Aaron officiated for the first time as high priest and his sons as regular priests. During the seven installation days, no fire descended from heaven to consume the sacrifices. Only when Aaron officiated, on the eighth day, did this miraculous sign of G‑d’s acceptance of the sacrifices occur.
When Aaron is Superior to Moses
וַיְהִי בַּיּוֹם הַשְּׁמִינִי קָרָא מֹשֶׁה לְאַהֲרֹן וּלְבָנָיו וגו': (ויקרא ט:א)
On the eighth day, Moses summoned Aaron and his sons. Leviticus9:1

Due to the limitations of our finite human minds, we cannot attain ultimate Divine consciousness on our own. G‑d therefore revealed Divinity in such a way that we can grasp it, by giving us the Torah. Once this had been accomplished, the next step was to prepare the world to absorb the G‑dliness that is inherent in the Torah, for without preparation on our part, Divine revelation cannot be absorbed into our being, and therefore cannot elevate us in any meaningful or lasting way.

G‑d gave us the Torah through Moses, but Aaron was the one who made society receptive to G‑dliness by inspiring the people to aspire to the spiritual life. It was therefore Aaron who completed the process of Divine revelation begun by Moses. The rites that Moses performed in the Tabernacle’s installation rites did not reveal G‑d’s presence; only those that Aaron performed accomplished this.

We all desire to feel G‑d’s presence in our lives. In order for this to occur, we must imitate Aaron: “love peace and pursue peace; love your fellow creatures and bring them close to the Torah.”1