Biblical origin of the holiday
The rites to be observed on the Day of Atonement are fully set forth in the sixteenth chapter of Leviticus (cf. Exodus 30:10; Leviticus 23:27-31, 25:9; Numbers 29:7-11). It was a most solemn fast, on which no food could be taken throughout the whole the day, and servile works were forbidden. It was kept on nineteenth day of Tischri, which falls in September/October. The sacrifices included a calf, a ram, and seven lambs (Numbers 29:8-11). But the distinctive ceremony of the day was the offering of the two goats.
The general meaning of the ceremony is sufficiently shown in the text. But the details present some difficulty. The Vulgate caper emissarius, "emissary goat", represents the obscure Hebrew word Azazel, which occurs nowhere else in the Bible. Various attempts have been made to interpret its meaning. Some have taken it for the name of a place where the man who took the goat away used to throw it over a precipice, since its return was thought to forbode evil. Others, with better reason, take it for the name of an evil spirit; and in fact a spirit of this name is mentioned in the Apocryphal Book of Henoch, and later in Jewish literature. On this interpretation—which, though by no means new, finds favour with modern critics—the idea of the ceremony would seem to be that the sins were sent back to the evil spirit to whose influence they owed their origin. It has been noted that somewhat similar rites of expiation have prevailed among heathen nations. And modern biblical critics, who refer the above passages to the Priestly Code, and to a post-Exilic date, are disposed to regard the sending of the goat to Azazel as an adaptation of a pre-existing ceremonial.
Evolution of the holiday in rabbinic writings
Yom Kippur today