There are a number of fast days throughout the year in Judaism.
25 hour fasts - from the start of the Jewish day (sunset the evening before) until nightfall on the day concerned.
- Yom Kippur - the Day of Atonement. Jewish date is the 10th of Tishri. Falls in September or October. A day spent in Synagogue praying for forgiveness.
- Tisha B'av. Jewish date is the 9th of Av. Falls in August usually. Possibly the saddest day in the Jewish calendar, commemorates the destruction of both Temples in Jerusalem and many other bad events in Jewish history.
Daily fasts - from sunrise until sunset only (therefore the ones in the winter are easier than the ones in the summer).
- Tsom Gedaliah. Jewish date is the 3rd of Tishri, the day after Rosh Hashanah. Falls in September or October. It commemorates the slaying of Gedaliah ben Ahikam, the governor of Judah appointed by Nebuchadnezzar, King of Babylon, after the destruction of Jerusalem.
- Asarah b'Tevet. Jewish date is the 10th of Tevet. Falls in December or January. It marks the beginning of the siege of Jerusalem by Nebuchadnezzar.
- Ta'anit Esther. Jewish date is the 13th of Adar, the day before Purim. Falls in March or April. Described in the biblical book of Esther. Queen Esther asked her uncle Mordecai to declare a fast for the Jews as a means of praying for her success with King Ahasuerus and against the evil Haman.
- Shivah Asar b'Tamuz. Jewish date is the 17th of Tamuz. Falls in June or July. This was the day the Romans breached the walls around Jerusalem, which led to the destruction three weeks later (Tisha B'Av) of the Second Temple in 70 C.E.
Some other fasts.
- Ta'anit B'Chorim - the Fast of the Firstborn. Jewish date is the 14th of Nissan, the day before Pesach (Passover). Traditionally first born males fast on this day to show we have sympathy with the Egyptians who lost their sons in the last of the 10 plagues, the Death of the First Born. Usually, though, a reason is found not to fast. This is simply because the meal on the first night of Pesach at the Seder feast is often held very late. The most common method is to complete learning a portion of Talmud after morning prayers, and on completion of learning any "section", a small meal is always held. After that, all present can eat for the rest of the day.
- Fasting for B'Hab. This isn't observed by many people nowdays. It involves fasting on the Monday, Thursday and Monday following the major festivals of Succot and Pesach. It is in case the celebreations of the festival caused you to do any sins that may be inappropriate for the nature of the festival.
- Wedding Day. Most religious Jews fast from when they get up on their wedding day until the ceremony itself. This is because the day is considered a personal Yom Kippur, on which they are forgiven their sins and start again as a married man or woman.
Jewish fasting is a complete fast - one is not allowed to eat or drink anything for the duration of the fast. Of course, if it would be dangerous for one to fast (say if you're diabetic), one mustn't fast. But the decision to eat or drink, especially on Yom Kippur (as the holiest day in the year) should be made after consultation with both a Rabbi and a Doctor.
The obligation to fast commences when a boy is Bar Mitzvah (13) and a girl is Bat Mitzvah (12). Most children would fast for one or two years before this, and even younger children, while they shouldn't fast, should still mark the day by not eating luxury foods, sweets or chocolate.
A huge majority of Jews fast on Yom Kippur, however religious they are. Only religious Jews fast on most of the other days.