Leviticus 16:4
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]. Leviticus 16:23-24
23-28. Aaron shall come into the tabernacle of the congregation, and shall put off the linen garments—On the dismissal of the scapegoat, the high priest prepared for the important parts of the service which still remained; and for the performance of these he laid aside his plain linen clothes, and, having bathed himself in water, he assumed his pontifical dress. Thus gorgeously attired, he went to present the burnt offerings which were prescribed for himself and the people, consisting of the two rams which had been brought with the sin offerings, but reserved till now. The fat was ordered to be burnt upon the altar; the rest of the carcasses to be cut down and given to some priestly attendants to burn without the camp, in conformity with the general law for the sin offerings (Le 4:8-12; 8:14-17). The persons employed in burning them, as well as the conductor of the scapegoat, were obliged to wash their clothes and bathe their flesh in water before they were allowed to return into the camp.