Exodus 29.35 says that seven days were required to consecrate Aaron and his sons as priests, and a few sentences later, the Lord said,

“Seven days you shall make atonement for the altar and sanctify it. And the altar shall be most holy. Whatever touches the altar must be holy” (Exodus 29.37).

The Lord works in sevens, at least when it comes to many of the things of man.

Seven days to consecrate a priest, and seven days to make atonement for the altar and to sanctify it.

Seven days was enough to get the job done. Six days would not have been enough. Eight days would have been too much. Seven days completed the task.