Charles Ellicott Commentary Leviticus 16:34

"And this shall be an everlasting statute unto you, to make atonement for the children of Israel because of all their sins once in the year. And he did as Jehovah commanded Moses." — Leviticus 16:34 (ASV)

And this shall be an everlasting statute. —Better, And this shall be a statute for ever, as the Authorized Version has it in Leviticus 16:29. Here, again, we have an instance of how the same phrase which occurs three times within four verses (Leviticus 16:31; Leviticus 16:34) is rendered in the Authorized Version by two different phrases, thus giving the English reader the idea that there is a difference in the original. The thrice-repeated phrase emphasizes the abiding nature of this law and indicates the solemnity of the day.

And he did as the Lord commanded Moses. —This means that Aaron performed the service for the first time according to the ordinances which the Lord communicated to Moses. A similar remark occurs at the first observance of the Passover . The solemn admonition, therefore, addressed to the priesthood at the beginning of this chapter , not to presume on their office, but to submit to the Divine regulations, was duly observed by the first high priest.

It may, however, also be designed to indicate that Aaron did not assume the dignity of the pontificate to exalt himself, but did so in obedience to the command God gave to Moses.