By Rabbi David Cohen:
יהי ביום השמיני קרא משה לאהרון ולבניו ולזקני ישראל
It was on the eighth day, Moshe summoned Aharon and his sons, and the elders of Yisrael. (9:1)
The command to bring the offerings was for Aharon HaKohen alone. Why were the Zekeinim, Elders, included in the summons? Rashi explains that Moshe Rabbeinu wanted the Elders to hear for themselves that Hashem had elevated Aharon to the position of Kohen Gadol. They should not suspect that Aharon had seized it for himself, or that Moshe had played favorites and given it to his older brother. Why was it necessary for Moshe to repeat himself? In Parashas Tzav (8:5), Moshe says to the congregation, “This is the thing that Hashem commanded to be done.” Rashi (to Tzav) explains that Moshe is actually saying, “You will see that everything that I do before you is upon the directive and command of Hashem and not for my honor or for the honor of my brother.”
Horav Moshe Feinstein, zl, explains the first time Moshe informed the nation that Aharon was Hashem’s choice for the position of Kohen Gadol, he conveyed to them that Aharon possessed the erudition, personal sanctity and ethical/moral character crucial for the role of Kohen Gadol. This, however, is not all a Kohen Gadol requires. He is the guiding spirit, and mentor of the nation. Erudition alone is not sufficient. He requires deep sensical knowledge as well.
After observing Aharon during the seven-day Inauguration period, Moshe told the people that Aharon was highly qualified to execute the functions of Kehunah Gedolah. This meant that not only was he spiritually fit and scholarly erudite, he was also a sensitive and pragmatic leader, capable of: communicating their spiritual obligations to the people; and dealing with each Jew’s spiritual challenge.
Shabbat Shalom