Atonement, Day of - (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 Lev. 16:3-10; 23:26-32; and Num. 29:7-11.
It was kept on the tenth day of the month Tisri, i.e., five days before the feast of Tabernacles, and lasted from sunset to sunset. (See AZAZEL.)