The Lord spake unto Moses. It is a matter of mere conjecture at what point of time this command was given. As it concerned the priests and their daily ministration, it would be natural to suppose that it was given at the time when the tabernacle service was set up, i.
e; at the precise point fixed by the first verse of the following chapter. That the command was given to Moses, and to Moses alone, and that after the consecration of Aaron to the high priesthood, serves to bring out into clear relief the relative position of the two.
Aaron and his sons alone, as the "official" representatives of the Lord, could bless in his name and put his name upon the people; but the formula of blessing was delivered to Aaron himself through Moses, as the "personal" representative of the Lord, the mediator of the old covenant.
ὁ νόμος … διαταγεὶς … ἐν χειρὶ μεσίτου (Galatians 3:19). Our Lord is both the Moses (Acts 3:22) and the Aaron (Hebrews 6:20)— ὁ μεσίτης and ὁ ἀρχιερεὺς—of this dispensation.