Yom Kippur [Heb.,=day of atonement], in Judaism, the most sacred holy day, falling on the 10th day of the Jewish month of Tishri (usually late September or early October). It is a day of fasting and prayer for forgiveness for sins committed during the year. Jews gather in synagogues on the Eve of Yom Kippur, when the fast begins, and return the following morning to continue confessing, doing penance, and praying for forgiveness. The most solemn of the prayers, Kol Nidre, is chanted on the Eve of Yom Kippur. Biblical origins are found in Leviticus, where the priestly ritual of atonement is described.