The culmination of the Yamim Noraim, Days of Awe, is the fast day of Yom Kippur, the Day of Atonement, the most important day in the Jewish liturgical calendar.
This is the day at the conclusion of which, according to tradition, God seals the Books of Life and Death for the coming year. The day is devoted to communal repentance for sins committed over the course of the previous year. Because of the nature of Yom Kippur and its associated rituals, it is the most solemn day in the Jewish calendar.
Yom Kippur is the day on which observers divorce themselves from the mundane world in which we live, in order to devote themselves to their relationship with the divine. Fasting is the most widespread manifestation of this devotion. Other examples include: refraining from washing, sexual relations and the wearing of leather which was seen as a sign of luxury in earlier times. It is traditional to dress in white on this day, symbolizing personal purity.
The liturgy of Yom Kippur is completely centered in the synagogue. It is traditional to wear a tallit, or prayer shawl, at all times in the synagogue on Yom Kippur; this is the only time during the year when the tallit is worn in the evening. There are more and longer services on this day than any other in the Jewish calendar. Yom Kippur is ushered in while it is still light out with a powerful and ancient prayer called Kol Nidrei (All Vows), in which the congregation asks that all vows made under duress during the coming year may be considered null and void before God.
In addition to the three daily services of Maariv (evening service), Shachrit (morning services), and Mincha (afternoon service), the Yom Kippur liturgy adds a special Musaf (additional) service. On Yom Kippur, Yizkor, the memorial service, is recited, as is the Avodah, a symbolic reenactment of the ancient priestly ritual for Yom Kippur. During the course of the holiday, a major component of the liturgy is the repeated communal confession of sins, the Viddui.
The day closes with an emotionally powerful service called Neilah, during which the liturgy imagines the gates of heaven closing at the end of the high holiday period. Neilah, during which it is traditional to stand since the ark is opened, ends with a long blast of the shofar or ram's horn, understood by many as signifying God's redemptive act in answer to true repentance.
During the holiday all thoughts are supposed to be centered on repentence. From Kol Nidrei to the repeated Viddui to Neilah, the day revolves around the theme of communal repentance for sins committed during the past year, in order that both the community and the individual be inscribed in the Book of Life for the coming year.