And it was on the eighth day, Moshe called Aharon and his sons and the elders of Klal Yisroel.
The Ohr Hachaim explains the nuances of this possuk. It is usually quite difficult for a person to accept that someone else receives something greater than himself. People are naturally egotistic and wish to stand at the head of the pack at all times. Two brothers are usually subconsciously more competitive than two strangers, and it is doubly difficult for a person to elevate his brother over himself. If a person has no choice and needs to elevate someone else, even if it is his own brother, he will still find it impossible to do so perfectly. He will procrastinate and push things off until he has no choice but to do his job. He may also not do it with the full attention to detail, and he will be careful to hide the ceremony as much as possible.
Moshe Rabbeinu showed that he was above all of these three issues, and he gave the Kehuna to his brother with a full heart.
וַיְהִי בַיּוֹם הַשְמִינִי – and it was on the eighth day. Immediately as the eighth day arrived, Moshe began this process of appointing his brother. He did not procrastinate or attempt to push things off as long as possible.
לְאַהֲרֹן וּלְבָנָיו – to Aharon and his sons. Not only did Moshe fulfill this with Aharon and elevate him to his lofty position, but he also did so with maximum attention to detail, choosing his sons and treating them exactly as the Torah commanded. Even though this underscored the fact that Moshe did not receive the Kehuna for himself and his generations of descendants, the priesthood was permanently granted to his brother Aharon; Moshe followed his obligations precisely.
וּלְזִקְנֵי יִשְרָאֵל – and to the elders of Klal Yisroel. Moshe made sure to do this job in public. He did not try and hide his shame and loss; he showed the world that Aharon and his sons would be the Cohanim from now and further.
All Moshe cared about was Hashem’s will; no personal emotion entered the equation. He was free of the subconscious motives of others, only wishing to serve his Creator.
