Wikipedia:

Yom Kippur (Hebrew: ˈjom kiˈpur ), also known as the Day of Atonement, is one of the holiest days of the year for Jews.

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 completes the annual period known in Judaism as the High Holy Days (or sometimes "the Days of Awe").

Yom Kippur is the tenth day of the month of Tishrei. According to Jewish tradition, God inscribes each person's fate for the coming year into a book, the Book of Life, on Rosh Hashanah, and waits until Yom Kippur to "seal" the verdict. During the Days of Awe, a Jew tries to amend his or her behavior and seek forgiveness for wrongs done against God (bein adam leMakom) and against other human beings (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 one's self 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 unique prayer dedicated to the special Yom Kippur avodah (service) of the Kohen Gadol in the Holy Temple in Jerusalem.

As one of the most culturally significant Jewish holidays, Yom Kippur is observed by many secular Jews who may not observe other holidays. Many secular Jews attend synagogue on Yom Kippur-for many secular Jews the High Holidays are the only recurring times of the year in which they attend synagogue, causing synagogue attendance to soar, and almost two-thirds fast.

General observances

Leviticus 16:29 mandates establishment of this holy day on the 10th day of the 7th month as the day of atonement for sins. It calls it the Sabbath of Sabbaths and a day upon which one must afflict one's soul.

Leviticus 23:27 decrees that Yom Kippur is a strict day of rest.

Five additional prohibitions are traditionally observed, as detailed in the Jewish oral tradition (Mishnah tractate Yoma 8:1)

The number five is a set number, relating to:

1. In the Yom Kippur section of the Torah, the word soul appears five times.
2. Soul, in the Torah is known by five separate names: soul, wind, spirit, living one and unique one.
3. Unlike regular days, which have three prayer services, Yom Kippur has five- Maariv, Shacharis, Mussaf, Minchah and Neilah
4. The Kohen Gadol rinsed himself in the mikveh five times on Yom Kippur.

The traditions are as follows:

1. No eating and drinking
2. No wearing of leather shoes
3. No bathing or washing
4. No anointing oneself with perfumes or lotions
5. No marital relations