John Calvin Commentary Numbers 8:20

"Thus did Moses, and Aaron, and all the congregation of the children of Israel, unto the Levites: according unto all that Jehovah commanded Moses touching the Levites, so did the children of Israel unto them." — Numbers 8:20 (ASV)

And Moses, and Aaron, and all the congregation—The Levites also are now inaugurated for the performance of their duties, but in their proper order, because their condition was inferior. Here it must be noted that the sons of Moses and their descendants were placed in this lower rank and excluded from all expectation of the priesthood.

For this reason, the ingratitude of the whole people, and especially of the tribe of Levi, was all the more shameful when they presumptuously sought the honor from which Moses had shut out his children forever. It was, therefore, no ordinary act of obedience on his part to execute what God had appointed regarding the Levites.

Aaron is mentioned here because he consecrated the Levites by virtue of his priesthood. As for the people, their consent is merely commended because they agreed to what was God's pleasure. But this virtue in them only increased their disgrace afterward when they sought to overthrow that divine decree of which they had approved.