When Did Aaron Become High Priest of Israel?

 Here's a question I was asked recently on social media:

Greetings brother Jay. I have been hashing over this high priest thing for a while now. I have yet to find chapter and verse saying Aaron was ever ordained as 'high priest' any where in Scripture. Your thoughts? Shalom!

At the beginning I don't think that "high priest" was a title so much as it was a relative position, much like saying "highest priest" or "senior priest".

In Exodus 40, God told Moses to bring Aaron and his sons and anoint them for the priesthood, but in v13, he separated out Aaron to be the first priest. Once Aaron was anointed and consecrated as priest, then Moses was to anoint his sons, thus ensuring that Aaron would have seniority, both in the family and in the priesthood. In fact, for a short period of time, Aaron was the only priest. Additionally, the high priest's uniform (begedi hakadosh) was given to Aaron, while his sons received tunics (ketunot).

Of course, after that second generation had passed away, seniority would become a cloudier issue. Scripture doesn't say how the high priest is to be determined after that. (I say "determined" rather than "selected" because the authority of precedence and seniority is inherent and not something that can be bestowed after-the-fact.)

The tl;dr version: I agree that Aaron isn't explicitly titled high priest at his ordination, but I think it's safe to assume by the order of events and the uniforms that were issued.

Exodus 40:12-15  Then you shall bring Aaron and his sons to the entrance of the tent of meeting and shall wash them with water  13  and put on Aaron the holy garments. And you shall anoint him and consecrate him, that he may serve me as priest.  14  You shall bring his sons also and put coats on them,  15  and anoint them, as you anointed their father, that they may serve me as priests. And their anointing shall admit them to a perpetual priesthood throughout their generations.