Yom Kippur is often considered the holiest day of the Jewish year. This day of fasting, prayer, and introspection begins with Kol Nidre, a prayer that asks for absolution from vows, and ends with the Neilah prayer. In between is a liturgy that runs the emotional gamut, from the solemn to the celebratory.
The culmination of the Yamin Noraim (Days of Awe) is the fast day of Yom Kippur (The Day of Atonement). 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.
A ritual for the expiation of sins was in existence already during biblical times. However, it was only during the Second Temple Period that Yom Kippur assumed central importance as a day of mourning and abstention. By the Rabbinic Period, it had become the most important day in the Jewish liturgical calendar, an importance that the day has retained until the modern period.
In the Community
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), Shaharit (morning services), and Minhah (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 a unique and 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.
Material excerpted from http://www.myjewishlearning.com/