The Sefer Ha’hinuch lists as the 185th Biblical command the Misva of the special Yom Kippur service which the Kohen Gadol would perform each year in the Bet Ha’mikdash on Yom Kippur, through which to earn atonement for the Jewish Paople.
The Sefer Ha’hinuch writes that this Misva was given as a great act of kindness on G-d’s part, offering us the opportunity to attain forgiveness for our sins through repentance. The Torah commands performing this service each year, the Sefer Ha’hinuch adds, in order to ensure that our sins do not accumulate to the point where the world will be deserving of destruction. The yearly observance of Yom Kippur is thus vitally important not only for each person individually, but also for the world at large, allowing it to continue existing.
The Sefer Ha’hinuch mentions in this context the tradition that the notion of an annual day of atonement existed already at the time of creation. In telling of the six days of creation, the Torah concludes its account of each day with the expression, “Va’yehi Ereb, Va’yehi Boker, Yom…” – “It was evening and it was morning, [such-and-such] day.” The days are referred to as “Sheni” (“second”), “Shelishi” (“third”), “Rebi’i” (“fourth”), and so on. The exception is the first day, which is referred to as “Yom Ehad” (“day one”), as opposed to “Yom Rishon” (“the first day”). The Rabbis understood the expression “Yom Ehad” as an allusion to “the singular day,” the special day chosen as the day on which to attain atonement – namely, Yom Kippur. Thus, the notion of Yom Kippur was conceived already at the earliest stages of the world’s creation. Already then, G-d designated Yom Kippur as a day uniquely suited for earning atonement and cleansing ourselves of our misdeeds.
The special sacrifices offered on Yom Kippur are in addition to the daily Tamid offering, which involves the offering of a sheep each morning and each afternoon; this sacrifice is offered even on Yom Kippur. Moreover, like every other Yom Tob, the occasion of Yom Kippur requires a Musaf offering. This offering, as the Torah tells us in Parashat Pinhas (Bamidbar 29:8-11), consists of a bull, a ram, seven sheep, and a goat as a Hatat (sin-offering). The meat of a Hatat must be eaten by the Kohanim; in the case of the Hatat brought as part of the Musaf sacrifice on Yom Kippur, the meat obviously cannot be eaten on Yom Kippur, when fasting is required, and so it is eaten at night, after the conclusion of the fast.
The special Yom Kippur sacrifices include a personal offering brought by the Kohen Gadol with his money: a bull as a Hatat, which is entirely burnt, and a ram offered as an Ola (burnt-offering). Additionally, the Kohen Gadol offers on behalf of the nation (from public funds) two goats – one which is sacrificed as a Hatat, and another which is sent to the desert and killed there (“Sa’ir La’azazel”). A ram as an Ola is also offered on the nation’s behalf. Altogether, excluding the goat sent to the desert, fifteen different sacrifices are offered on Yom Kippur.
All Avodot (services) performed in the Bet Ha’mikdash on Yom Kippur must be performed specifically by the Kohen Gadol. A special Halacha requires the Kohen Gadol to be married in order to perform the Yom Kippur service.
This Misva, of course, applies only in the times of the Bet Ha’mikdash. However, the Sefer Ha’hinuch writes that nowadays, we conduct the Yom Kippur service through prayers and supplications, humbly begging G-d for forgiveness, and these prayers are offered in lieu of the special atonement sacrifices which we are presently unable to offer.