Adam Clarke on Leviticus 9:1

On the eighth day - This was the first day after their consecration, before which they were deemed unfit to minister in holy things, being considered as in a state of imperfection. "All creatures," says Ainsworth, "for the most part were in their uncleanness and imperfection seven days, and perfected on the eighth; as children by circumcision, Lev 12:2, Lev 12:3; young beasts for sacrifice, Lev 22:27; persons that were unclean by leprosies, issues, and the like, Lev 14:8-10; Lev 15:13, Lev 15:14; Num 6:9, Num 6:10. So here, the priests, until the eighth day, were not admitted to minister in their office."