The Lord spoke to Moses after the death of two of Aaron's sons when they approached the presence of the Lord and died.
References for Leviticus 16:1
The Lord said to Moses: "Tell your brother Aaron that he may not come whenever he wants into the holy place behind the veil in front of the mercy seat on the ark or else he will die, because I appear in the cloud above the mercy seat.
References for Leviticus 16:2
"Aaron is to enter the [most] holy place in this way: with a young bull for a sin offering and a ram for a burnt offering.
References for Leviticus 16:3
He is to wear a holy linen tunic, and linen undergarments are to be on his body. He must tie a linen sash [around him] and wrap his head with a linen turban. These are holy garments; he must bathe his body with water before he wears them.
References for Leviticus 16:4
He is to take from the Israelite community two male goats for a sin offering and one ram for a burnt offering.
References for Leviticus 16:5
"Aaron will present the bull for his sin offering and make atonement for himself and his household.
Next he will take the two goats and place them before the Lord at the entrance to the tent of meeting.
After Aaron casts lots for the two goats, one lot for the Lord and the other for Azazel,
References for Leviticus 16:8
he is to present the goat chosen by lot for the Lord and sacrifice it as a sin offering.
References for Leviticus 16:9
But the goat chosen by lot for Azazel is to be presented alive before the Lord to make purification with it by sending it into the wilderness for Azazel.