Yom Kippur, also known in English as the Day of Atonement, is the most solemn and important of the Jewish holidays. Its central themes are atonement and repentance. Jews traditionally observe this holy day with a 25-hour period of fasting and intensive prayer, often spending most of the day in synagogue services. Yom Kippur is the tenth and final day of the Ten Days of Repentance which begin with Rosh Hashanah.

According to Jewish tradition, God inscribes each person's fate for the coming year into a "book" on Rosh Hashanah and waits until Yom Kippur to "seal" the verdict. During the Ten Days of Repentance, a Jew tries to amend his behavior and seek forgiveness for wrongs done against God (bein adam leMakom) and against his fellow man (bein adam lechavero). The evening and day of Yom Kippur are set aside for public and private petitions and confessions of guilt (Vidui). At the end of Yom Kippur, one considers himself absolved by God. The Yom Kippur prayer service includes several unique aspects. One is the actual number of prayer services. Unlike a regular day, which has three prayer services (Ma'ariv, the evening prayer; Shacharit, the morning prayer; and Mincha, the afternoon prayer), or a Shabbat or Yom Tov, which have four prayer services (Ma'ariv; Shacharit; Musaf, the additional prayer; and Mincha), Yom Kippur has five prayer services (Ma'ariv; Shacharit; Musaf; Mincha; and Ne'ilah, the closing prayer). The prayer services also include a public confession of sins (Vidui) and a reenactment of the special Yom Kippur avodah (service) of the Kohen Gadol in the Holy Temple in Jerusalem. (via Wikipedia)