The Seventeenth of Tammuz is a public fast day to commemorate the first breach of Jerusalem's walls in the First and Second Temples. It is a daytime fast in which we refrain from eating and drinking from sunrise to sunset. This year it falls on Sunday, July 21, from 4:01am to 9:10pm. It is also the beginning of the mourning period known as the Three Weeks, which culminate in another fast day, Tisha b'Av (9th of Av).
There are five tragedies associated with the day:
1. In the Wilderness, Moses broke the Tablets.
2. First Temple, the Babylonians breached the walls.
3. Second Temple, The Romans breached the walls.
4. Apostomos, a Roman general, publicly burned a Torah scroll.
5. An idol was placed in the Temple by Apostomos, or some say it was Menashe, the idol-worshipping king of Judah.
