In 423 BCE, Nebuchadnezzar's troops breached the walls of old Jerusalem and entered the city. Four weeks later, the Holy Temple was destroyed, and the Jews were exiled to Babylon. Originally, a day of fasting and mourning was observed on the 9th of Tammuz. Seventy years later, however, when the Second Temple was built, the fast was abolished and the day was turned into a holiday. Some 500 years later when Jerusalem fell on the 17th of Tammuz -- prior to the destruction of the Second Temple -- the Sages decreed the 17th of Tammuz as a fast day to commemorate both tragedies.