The high priest had to make elaborate preparations for entering the holy of holies by cleansing himself spiritually and physically. The offerings he made were a bull as a sin offering and a ram as a burnt offering, both to cover his own sins (cf. Hebrews 5:3). He also had to wear a special uniform, not the ornate garments that he usually wore to carry out his regular duties. This uniform consisted of four white garments and made him appear more as a slave than as a king. This dress was even simpler than that worn by the other priests (cf. Exodus 39:27-29). [source][source][source]
"Among his [1] fellow men his dignity as the great mediator between man and God is unsurpassed, and his splendid clothes draw attention to the glory of his office. But in the presence of God even the high priest is stripped of all honor: he becomes simply the servant of the King of kings, whose true status is portrayed in the simplicity of his dress [2]." [3][source]
". . . elaborate garments might have detracted from the somberness of the occasion, when atonement for sin was the basic concern." [4][source]
Aaron had to wash his body, symbolizing his cleanness. He also offered two goats as a sin offering and another ram as a burnt offering for the Israelites. The high priest entered the holy of holies only once each year on the Day of Atonement to make these special sacrifices (cf. Hebrews 9:7). [source][source][source]