Leviticus 16 opens by recalling the deaths of Nadab and Abihu, who came near to God carelessly. God told Moses that Aaron must not come at just any time into the Most Holy Place, behind the veil, before the mercy seat. The holiness of God was real, and approaching Him required His appointed way. That way was the Day of Atonement, held once a year on the tenth day of the seventh month.
Aaron set aside his glorious garments and wore plain holy linen. He offered a bull for himself and his household, and he brought burning incense so that a cloud would cover the mercy seat. He then cast lots over two goats. The goat for Yahweh was a sin offering for the people, and its blood was sprinkled on and before the mercy seat. Aaron then laid both hands on the head of the live goat, confessed over it all the iniquities of Israel, and sent it into the wilderness by the hand of a ready man.
The people were to rest and afflict their souls, humbling themselves before God. The chapter ends: This shall be an everlasting statute for you, to make atonement for the children of Israel once in the year because of all their sins. Hebrews 9 and 10 explain that the yearly repetition showed the sacrifices could not finally remove sin. Christ entered heaven itself once for all, by His own blood, obtaining eternal redemption. What Israel saw in a yearly picture, Jesus accomplished forever.