“With this shall Aaron come into the holy place, with a young bull ox for a purification for sin offering, and a ram for a whole burnt offering.”
On this day, after the morning whole burnt offering (a lamb of the first year) had been offered with its accompanying grain offering, Aaron's approach to Yahweh had to commence with offerings for himself and the priests. These would consist of a young bull ox for a purification for sin offering and a ram for a whole burnt offering. He must make sacrifices first for himself (Hebrews 5:3; Hebrews 9:7). He too was a sinner in need of atonement.
How much different was this from our great High Priest, the Lord Jesus Christ, Who was without sin, Whose perfections and Whose perfect life and Whose total obedience fitted Him for His office with no need of sacrifice (Hebrews 7:26-27).