Yon Kippur is the most solemn holy day in Judaism. It is a day of atonement (prayer, repentance and fasting) observed in some way by nearly all Jewish people. Orthodox Jews refrain completely from all food and drink on Yom Kippur.
Before the destruction of the Temple in Jerusalem, there was an elaborate ceremony of confession and repentance led by the high priest and including the sacrifice of a goat (the scapegoat).
The central prayer of Yom Kippur is the Kol Nidre, or "all vows". This Aramaic prayer is an appeal to God to forgive vows unfulfilled that do not affect any other person. Vows affected include only those made to oneself, or between oneself and God. Anti-semites have tried to use the prayer to malign Jews as untrustworthy.