Matthew Poole and continuators on Leviticus 9:1

1 AND it came to pass on the eighth day , that Moses called Aaron and his sons, and the elders of Israel .

To wit, from the first day of his consecration, or when the seven dayes of his consecration were ended, chap. 8. 33, 35. as appears from Exod. 29. 30. &c. Ezek. 43. 27. The eighth day is famous in Scripture for the perfecting and purifying both of men and beasts. See Levit, 12. 2, 3. and 14. 8, 9, 10. and 15. 13, 14. and 22. 27.

All the congregation were called to be witnesses of Aarons instalment into his office, to prevent their murmurings and contempt, which being done, the Elders were now sufficient to be witnesses of Aarons first execution of his office.