Introduction to the Day of Atonement

Leviticus 16:1-2

And Jehovah spake unto Moses, after the death of the two sons of Aaron, when they drew near before Jehovah, and died;

and Jehovah said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the veil, before the mercy-seat which is upon the ark; that he die not: for I will appear in the cloud upon the mercy-seat.