During the year, any kohen could perform the avoda, offering the sacrifices and incense, and preparing the menora for lighting. However, due to the great sanctity of Yom Kippur, only the Kohen Gadol was permitted to perform these duties (Yoma 32b; MT, Laws of the Yom Kippur Service 1:2).
The Kohen Gadol offered three types of sacrifices on Yom Kippur. The first category included the daily temidim – two lambs, one offered in the morning, before the rest of the sacrifices, and the other offered in the afternoon as the last of the day’s sacrifices. This category also included the offering of the incense (which was done on the incense altar twice daily, morning and afternoon), as well as preparing and lighting the menora. The second category was the musaf offerings, akin to those offered on Rosh Ḥodesh and the holidays. On Yom Kippur, these consisted of a bull, a ram, and seven lambs for burnt offerings, plus one goat for a sin offering. The third category was specific to Yom Kippur. It comprised a bull for a sin offering to atone for the Kohen Gadol and the rest of the kohanim, a ram as a burnt offering (both of which the Kohen Gadol paid for himself), and two goats to atone for Israel: one goat was a sin offering, and the other was sent out into the wilderness.
The Kohen Gadol had to be married while serving on Yom Kippur, as we read, “‘To make expiation for himself and for his household’ (Vayikra 16:6). ‘His household’ refers to his wife” (Yoma 13a). This is despite the Kohen Gadol’s obligation to separate from his wife for a week before Yom Kippur, to purify and sanctify himself in preparation for the avoda. The reason he was required to be married is that someone who is not married is considered incomplete (Yevamot 63a), lacking joy, blessing, goodness, Torah, protection, and peace (ibid. 62b). The Kohen Gadol had to have one wife only; if he had two wives, he was disqualified from serving (Yoma 13a). For only within a monogamous relationship can ideal unity and love be achieved. Once the Kohen Gadol experienced this unity with his wife, he was also able to unite and connect the Jewish people with their Father in heaven.
An alternate Kohen Gadol was designated; he would step in should the Kohen Gadol become tamei or die (Yoma 2a; MT, Laws of the Yom Kippur Service 1:2-3, and Laws of the Temple’s Vessels and Its Workers 5:10).