leviticus 16:3 Thus shall Aaron come into the holy place: with a young bullock for a sin offering, and a ram for a burnt offering. - Free Bible Online

      3, 4. Thus shall Aaron come into the holy place--As the duties of the great day of atonement led to the nearest and most solemn approach to God, the directions as to the proper course to be followed were minute and special.
      with a young bullock . . . and a ram--These victims he brought alive, but they were not offered in sacrifice till he had gone through the ceremonies described between Le 16:3-11. He was not to attire himself on that occasion in the splendid robes that were proper to his sacred office, but in a plain dress of linen, like the common Levites, for, as he was then to make atonement for his own sins, as well as for those of the people, he was to appear in the humble character of a suppliant. That plain dress was more in harmony with a season of humiliation (as well as lighter and more convenient for the duties which on that occasion he had singly to perform) than the gorgeous robes of the pontificate. It showed that when all appeared as sinners, the highest and lowest were then on a level, and that there is no distinction of persons with God [Ac 10:34].

JFB.