It was on this day after "the week of training" for Aaron and his sons that God said to Moses: "Thinkest thou that thou are to be high priest because thou hast been attending to priestly duties during this week? Not so, call Aaron and announce to him that he has been appointed high priest, and at the same time call the elders and in their presence announce his elevation to this dignity, that none may say Aaron himself assumed this dignity." Following the example of God, who on Sinai distinguished Aaron before all others, saying, "And thou shalt come up, thou and Aaron with thee, but let not the priests and the people break through," Moses went first to Aaron, then to Aaron's sons, and only then to the elders, to discuss with them the preparations for the installation of Aaron into office.
Priest — Aaron in the Days of Moses
Legends of the Jews 3:70