The 17th of Tammuz is a fast day which commemorates the breaching of the walls of Jerusalem, leading to the destruction of the Temple. It also marks other tragedies: the destruction, by Moshe, of the tablets of the Ten Commandments, due to the sin of the golden calf;   the  ceasing of the daily sacrifice in the Temple; the burning of a Torah scroll by the Roman Apostomus; and the erection of an idol in the Temple. There is some difference of opinion about whether the breaching of the city walls took place in the period of the destruction of the first or second temples, or both.

This fast lasts from dawn to nightfall. Eating and drinking is forbidden for adults. People who are ill, and pregnant or nursing women, may be exempt from the fast, and should consult a Rabbi. 

The 17th of Tammuz is the first day of a three-week period of mourning, which comes to a climax on the ninth of Av, when we fast for a full day, from sundown to the next day's nightfall, mourning the destruction of both temples. During this three week period many refrain from  parties, weddings, and the like.