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Fasting and asceticism

The Torah commands Jews to "afflict themselves" (ve'initem et nafshoteichem) on Yom Kippur. While these verses do not explicitly mention the form of affliction, the phrase "afflicting oneself" frequently appears elsewhere in connection with fasting or lack of food, and public fast days for repentance were a common practice in Biblical times. According to the Jewish oral tradition, the Yom Kippur "affliction" consists of the following five prohibitions:

In traditional custom, the fast is required of males over age 13 and females over age 12. However, fasting is waived in the case of dangerous medical conditions (pikuach nefesh), and in such a case one is actually required to break the fast. Just as it is a mitzvah to fast on Yom Kippur, it may also be a mitzvah to eat or drink on Yom Kippur to safeguard a person's health. In such situations, though, it is preferable (if the medical situation allows for it) to consume only small amounts of food or drink at a time.

Fasting, along with the other restrictions, begins at sundown, and ends after nightfall the following day. One should add a few minutes to the beginning and end of the day, called tosefet Yom Kippur, lit. 'addition to Yom Kippur'.

Yom Kippur is the only day when fasting is permitted on Shabbat.