The Day of Atonement (Leviticus 16)
While the term "Day of Atonement" is nowhere mentioned in this chapter, we know that this Holy Day is the one referred to here based on the fact that the events described were to transpire on the 10th day of the seventh month (verse 29). And according to Leviticus 23:27, this is the date of the Day of Atonement. There is, of course, a clear "atonement" theme running throughout Leviticus 16, that very word being used 16 times in the chapter. Further explanation of the rich meaning behind the actions that were carried out on this day is given in the supplementary reading.
"Supplementary Reading: "The Day of Atonement: Removal of Sin's Cause and Reconciliation to God," God's Holy Day Plan: The Promise of Hope for All Mankind, pp. 38-43.