The 10th day of Tishri (September-October) is observed as the Day of Atonement and is also known as Yom Kippur. Yom Kippur is the Jewish day of repentance, considered to be one of the holiest and most solemn days of the year. Its central theme is atonement and reconciliation, hence it is known as the day of atonement. It is the day, when God is expected to seal the judgment reached on the individual lives, in the light of their thoughts, words and deeds in the preceding year. After kindling the Yom Kippur lights at home, on the eve of the festival, the family goes to the synagogue for the prayer service. Men (and some reform and conservative women) don a tallit (four-cornered prayer garment) for evening prayers, the only evening service of the year in which this is done. Many married men also wear a kittel, or white shroud-like garment, which symbolizes inner purity.

Prayer services begin with the prayer known as "Kol Nidre," which must be recited before sunset, and follows with the evening prayers (ma`ariv or arvith), which include an extended Selichot service.The morning prayer service is preceded by litanies and petitions of forgiveness called selichot; on Yom Kippur, many selichot are woven into the liturgy. The morning prayers are followed by an added prayer (musaf) as on all other holidays, followed by mincha (the afternoon prayer) and the added ne`ilah prayer specifically for Yom Kippur. Yom Kippur comes to an end with a recitation of Shema Yisrael and the blowing of the shofar, which marks the conclusion of the fast.

The Torah is read during the morning and afternoon prayers; the Book of Jonah is read as the haftarah in the afternoon. Depending on the nusach (version) of the prayers, some communities pray continuously from morning until nightfall, while some include a short break. Every prayer includes the vidduy (confession). This service is a solemn declaration absolving all members of the faith from the unfulfilled vows unwittingly or impulsively made to God by man. Vows, promises and undertakings between man and man cannot, however, so be annulled, such promises and undertakings must in any case be honoured.

A strict fast is observed from sunset to nightfall of the next day; eating, drinking, marital relations, use of cosmetics and toiletries are strictly forbidden. Total abstention from food and drink usually begins half-an-hour before sundown (called "tosefet Yom Kippur," the "addition" of fasting part of the day before is required by Jewish law), and ends after nightfall the following day. Although the fast is required of all healthy adults, fasting is specifically forbidden for anyone who might be harmed by it.

However, there is a commandment to eat a large and festive meal before Yom Kippur starts after the mincha prayer. Virtually all Jewish holidays involve a ritual feast; in the case of Yom Kippur, the meal that precedes the holiday supposedly makes the fast even more strenuous, thereby fulfilling the injunction "you shall afflict yourselves". Traditional foods consumed during that meal include kreplach and rice. Many others also have a custom to eat another meal before that, consuming fish. Also, many orthodox men immerse themselves in a mikvah.

Yom Kippur is observed in different ways in different Jewish communities. Some Jews refer to this holiday as "the White Fast." Consequently, many Jews have the custom of wearing only white clothing on this day, to symbolize their "white" (pure) desire to free themselves from sin. Others, while acknowledging the origins of the holiday as a day of rejoicing, tend to take a more somber, solemn attitude to the day.