Charles Ellicott Commentary Leviticus 16:3

"Herewith shall Aaron come into the holy place: with a young bullock for a sin-offering, and a ram for a burnt-offering." — Leviticus 16:3 (ASV)

Thus shall Aaron come. —A better rendering is, With this shall, etc.; that is, with the following sacrifices, ritual, vestments, and so on, he shall approach the most holy place, after previously offering the daily morning sacrifice and performing the ordinary daily service. During the performance of the morning service, the high priest, at the time of Christ, wore the golden vestments. He changed these for the white garments before he began the special ritual prescribed for this day.

With a young bullock for a sin offering. —This bullock had to be in its second year , and the high priest had to buy it with his own money. It had to be his own property because the victim was to expiate his own sins, since he, like the lowliest sinner, required Divine mercy and forgiveness, though, due to his high office, he had to bring a more costly sacrifice.