(Originally, the fast was observed on the Ninth of Tammuz since that was the day Jerusalem fell prior to the destruction of the First Temple in 586 BCE. However, after Jerusalem fell on the 17th of Tammuz - prior to the destruction of the Second Temple - the Sages decided upon a combined observance for both tragedies, the 17th of Tammuz.)
Five major catastrophes occurred in Jewish history on the 17th of Tammuz:
(1) Moses broke the tablets at Mount Sinai, in response to the building of the Golden Calf.
(2) The daily offerings in the First Temple were suspended during the siege of Jerusalem in the 5th century BCE.
(3) Jerusalem's walls were breached, prior to the destruction of the Second Temple in 70 CE.
(4) Prior to the Great Revolt, the Roman general Apostamos burned a Torah scroll -- setting a precedent for the horrific burning of Jewish books throughout the centuries.
(5) An idolatrous image was placed in the Holy Temple -- a brazen act of blasphemy and desecration.
From more on this date, click here.