Leviticus 16
The Day of Atonement regulations are introduced in the shadow of Nadab and Abihu's deaths: the high priest may enter the Most Holy Place only once a year, on the tenth of the seventh month, and only with the prescribed ceremony. Aaron enters wearing simple white linen (not the elaborate high priestly vestments), takes a bull for his personal sin offering and a ram for his burnt offering, and burns incense in the Most Holy Place to create a protective smoke-cloud before the mercy seat. He then applies the bull's blood and the Lord's-lot goat's blood to the mercy seat with sevenfold sprinkling, cleansing the Most Holy Place, the Holy Place, and the burnt offering altar from the accumulated year's contamination. He lays both hands on the live Azazel goat, confesses all Israel's sins over its head, and sends it into the wilderness — carrying the community's sin burden away permanently. The day requires the community's self-denial and complete work cessation, and is declared a lasting ordinance: atonement once a year for all the sins of the Israelites. Hebrews 9–10 identifies this chapter as the fullest Old Testament type of Christ's priestly work.
Leviticus 16:1
The Lord spoke to Moses after the death of the two sons of Aaron who died when they approached the Lord. The Day of Atonement regulations are introduced in the shadow of the deaths of Nadab and Abihu in Leviticus 10. The connection between the sons' deaths and the Yom Kippur regulations is the connection between unauthorized approach and authorized approach: the deaths of those who approached the divine presence improperly create the context for the regulations governing the one authorized annual approach to the Most Holy Place. The connection is not punitive but pedagogical: the tragedy teaches the necessity of the prescribed approach.
Leviticus 16:2
The Lord said to Moses: tell your brother Aaron that he is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or else he will die. For I will appear in the cloud over the atonement cover. The fundamental prohibition that the Day of Atonement regulations address: the high priest cannot enter the Most Holy Place at will — not whenever he chooses. The consequence is death; the reason is the divine presence in the cloud over the mercy seat. The God who appears in the cloud at the mercy seat is the God whose presence is both the goal of approach and the danger of unauthorized approach. Hebrews 9:7 says only the high priest entered the inner room, and only once a year — the once-a-year limitation of Leviticus 16:2 is the temporal expression of the authorized approach's restriction.
Leviticus 16:3
This is how Aaron is to enter the Most Holy Place: he must first bring a young bull for a sin offering and a ram for a burnt offering. The authorized entry into the Most Holy Place begins with the high priest's personal offerings: a young bull for his sin offering and a ram for his burnt offering. Before Aaron can enter the Most Holy Place, he must be atoned for. Before the mediator of Israel's atonement can approach the divine presence, he must himself be covered. The bull and the ram are the same animals that inaugurated the priestly ministry in Leviticus 9:2. The high priest's personal offerings are the foundation of the entire Day of Atonement ceremony.