Commentary for Exodus 28:1

ואתה הקרב אליך AND THOU TAKE TO THEE after the work of the Tabernacle is finished.

AND THOU BRING THOU NEAR AARON THY BROTHER, AND HIS SONS WITH HIM … THAT THEY MINISTER UNTO ME IN THE PRIEST’S OFFICE, EVEN AARON, NADAB AND ABIHU, ELEAZAR AND ITHAMAR, AARON’S SONS. The reason for mentioning Nadab and Abihu, Eleazar and Ithamar [when it mentioned already “and his sons”] is that Moses should not think that by anointing the father to minister as priest, his sons would automatically become priests; instead he had to initiate them personally into the priesthood. Thus Phinehas [the son of Eleazar] and others already born were excluded [from the priesthood], for only these four sons who were anointed with Aaron, and their children born to them henceforth, were appointed as priests.

ואתה הקרב אליך את אהרון אחיך, "And you shall bring near to you your brother Aaron, etc." We must understand the words: "and you- bring near- to you," in light of the statement in Shemot Rabbah 3,17 according to which it was G'd's original plan to appoint Moses as High Priest. This was changed due to Moses' repeated refusal to accept the role assigned to him by G'd which angered G'd. As a result (Exodus 4,14), G'd told Moses that his brother Aaron who had merely been a Levite up to that point, had now been promoted to be a High Priest. When G'd commanded Moses at this point to perform the ceremonies required for Aaron to assume the office of High Priest, He told Moses that he had to make his own contribution to this ceremony so that he would not be perceived as begrudging Aaron an office which had originally been intended for him. In fact, the appointment of Aaron to this position would serve as atonement for Moses who had resisted G'd's invitation to become leader of the Jewish people at that time.