Why did Aaron have to offer a bullock for himself on the Day of Atonement?
Aaron had to offer the bullock as a sin offering for himself because, although he was Israel’s high priest, he was a sinner who sinned like everyone else. Before he could begin the Day of Atonement duties, he first had to make atonement for himself and his household.
The teaching explains that before Aaron could begin the Day of Atonement, he had to wash repeatedly, depicting a complete physical cleansing, and then put on the linen undergarments and holy garments. He then took two young goats and a ram. In verse 6, Aaron was to offer his bullock of the sin offering for himself and make atonement for himself and for his house. Even though he was Israel’s high priest, he was a sinner and sinned just as anyone else. So, to begin the high priest’s role, Aaron had to kill this first sacrificial animal for himself and his family. This contrasts with Christ, who went in sinless even though He took our sin upon Himself. Aaron would then take the blood of that offering and make his way, stopping at the laver of cleansing, and take coals from off the altar to put on the altar of incense, with the smoke serving as his protection or covering.
And Aaron shall offer his bullock of the sin offering, which is for himself, and make an atonement for himself, and for his house.