The meaning of Atonement, Day of in the Bible
(From Easton's Bible Dictionary)

the great annual day of humiliation and expiation for the sins of the nation, "the fast" (Acts 27:9), and the only one commanded in the law of Moses. The mode of its observance is described in Leviticus 16:3-10; 23:26-32; and Numbers 29:7-11.

See more on the meaning of Atonement, Day Of in the Bible:
Atonement, Day Of {International Standard Bible Encyclopedia}

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