SHIVA ASAR B'TAMMUZ
"On this date in the year 423 BCE, the daily sacrificial offerings were discontinued in the run up to the destruction of the first temple. In the year 69 BCE Jerusalem’s walls were breached, which resulted in the destruction of the second temple. Finally, the Roman military leader Apostomus burned a Torah scroll, possibly around 50 CE. This may have contributed to the Bar Kokhba revolt, the last war between the Romans and the Jews between 132 and 135 CE.
The 17th of Tammuz marks the start of the “Three Weeks” (Bein HaMetzarim), which is a period of mourning marking the destruction of both the First Temple and the Second Temple in Jerusalem. During the “Three Weeks”, it is customary to spend extra time studying Jewish law, to give extra charity and not to hold joyous celebrations, such as weddings, or wear new clothes."