In Exodus 29, the Lord told Moses how to prepare Aaron and his sons for the priesthood, and in verse 35, the Lord indicated the time involved,
“Thus you shall do to Aaron and his sons, according to all that I have commanded you. Seven days you shall consecrate them” (Exodus 29.35).
Some people believe there is no significance to numbers in the Bible, even the number seven, but this text shows that the Lord wanted the Levitical priests consecrated over a seven day period and you know that was not simply incidental, but the Lord had His own purpose or purposes in mind.
In many cases in the Bible seven shows completion, and that seems to be the sense here; that it would take seven days, along with the activity of those days, to complete the consecration of Aaron and his sons as high priests.