In Judaism, a fast day or ta’anit is a day to refrain from all food and drink, including water. There are several minor fast days, lasting from dawn until dark, and two major ones, Tish’a b’Av and Yom Kippur, which last 25 hours, from sunset on the evening before the fast day until after dark on the day itself. The purpose of fasting may be an expression of mourning (as in Tish’a b’Av, which commemorates the destruction of both Temples, and the minor fasts of Gedaliah, the Tenth of Tevet and the Seventeenth of Tammuz, all of which are related to events leading to that destruction), supplication (such as the minor Fast of Esther) or repentance, as in Yom Kippur. Orthodox Jews observe both the minor and the major fast days, whereas Reform Jews generally restrict themselves to observing Yom Kippur only, and Conservative Jews, as usual, fall somewhere in between.
Yom Kippur is the only fast day mentioned in the Torah. In Leviticus 23:27, G-d tells Moses to instruct the people that they shall “afflict their souls” on this day. The Talmud interpreted this as meaning refraining from food and drink, as well as four other restrictions (on bathing, sexual relations, anointing with oil or cream, and wearing leather shoes). Even secular Jews who do not observe any other rituals will sometimes keep the fast of Yom Kippur.
On Yom Kippur, we spend the day in prayer, divorcing ourselves from our bodily cravings and becoming as close as possible to the angels. Refraining from food and drink helps us focus on spiritual matters and commune with our souls on this holy day.