Introduction to the Day of Atonement

Leviticus 16:1-2

And Jehovah speaketh unto Moses, after the death of the two sons of Aaron, in their drawing near before Jehovah, and they die;

yea, Jehovah saith unto Moses, `Speak unto Aaron thy brother, and he cometh not in at all times unto the sanctuary within the vail, unto the front of the mercy-seat, which is upon the ark, and he dieth not, for in a cloud I am seen upon the mercy-seat.