Day Of Atonement
From the Ecylopaedia Judaica

Yom ha Kippurim, one of the "apointed seasons of the Lord, holy convocations" a day of fasting and atonement, occuring on the tenth month of Tishri. It is the climax of the "Ten day of Penitence" and the most important day in the liturgical year.
In the Bible
all manner of work is forbdden on the Day of Atonement, as it is on the Sabbath (being likewise called "a Sabbath of solemn rest" and the soul is to be "afflicted from evening of the ninth day of the seventh month until evening of the morrow", the punishment for transgressing these commandments destruction and extirpation (
Lev. 16:29-31 ; 23:27-32 ; Num. 29:7).
Special additional offerings were to be brought (Num. 29:8-11), and apart from these, a ceremony peculiar to the day ( see Avodah) was solemnized in the Temple (Lev. 16:1-34). The essence of the day and the reasons for the ceremony are expressed by the verse: "For on this day shall attonement be made for you; from all your sins shall you be clean before YEHOVAH". (Lev. 16:30)
In the Jubilee year the shofar is to be sounded on the Day of Atonement to indicate the setting free of slaves and the restoration of the fields to thier ancestral owners. (
Lev 25:9-10)

In the second temple period. The ritual performed by the high priest in the Temple was the central feature of the Day of Atonement (see Avodah ; Sacrfice). When the high priest, representative of the people (Yoma 1:5), "entered where he was entered and stood where he stood" (5:3) he himself was enveloped in awe and holiness, and mystery ; while when he had come out, he resembled, in his majesty, "a bright star emerging from between the clouds" (Ecclus. 50:6 ff)