After the construction of the Mishkan was completed, it had to be consecrated. The Gemara explains that Moshe had originally been tapped to be the Kohen Gadol as well, but lost this privilege when he resisted God’s overtures to save the Jewish People at the beginning of the Exodus story.
So for one week, Moshe served as a sort of “soft opening,” effectively serving as the Kohen Gadol. After those seven days, God told Moshe to instruct Ahron how to perform the Kohen’s duties:
אַתָּה הַקְרֵב אֵלֶיךָ אֶת־אַהֲרֹן אָחִיךָ וְאֶת־בָּנָיו אִתּוֹ מִתּוֹךְ בְּנֵי יִשְׂרָאֵל לְכַהֲנוֹ־לִי… – You shall draw close your brother, Aaron, with his sons, from among the Jewish People, to serve Me as priests… (28:1)
Moshe had to serve in the capacity of Kohen Gadol for a short time, and then pass the methods on.
But why not just give the job directly to Ahron from the outset?
The Ohr Hachaim suggests that Moshe had to serve for a short time so that he would see what he lost by not eagerly grabbing the opportunity as soon as possible. Moshe had to gather Ahron’s family to teach them – הַקְרֵב אֵלֶיךָ – but the root of קרב is cognate to sacrifice. Moshe had to come close to see what he gave up – הַקְרֵב אֵלֶיךָ.
It’s worthwhile to note that when this transition period ended, the Torah marks Moshe’s final act in the cantillation marks with a Shalsheles, a rare note which translates as “chain.” The Shalsheles sounds like what it conveys, a wavering and faltering hesitation before finally letting go, breaking the chain as it were, and now Moshe had learned what a vital position Ahron held.
When it comes to essential things, it’s worth understanding what the opportunity is and what its associated costs and benefits will be before making a decision.
While we can’t say yes to everything, we can certainly give it some thought before saying no!