“On the eighth day Moses called Aaron and his sons, and the elders of Israel.” Leviticus 9:1
We are finely dedicating the Sanctuary, and it is on the eighth day.
But there are only 7 days in the week?
The eight day is for the events that are above nature. Like the covenant of circumcision, not natural to make a covenant with God, so it is on the 8th day, the day above nature.
Not natural to dedicate a home to God in our midst, so it is on the eighth day.