17th of Tammuz: History, Laws and Customs

 https://www.chabad.org/library/article_cdo/aid/479885/jewish/The-17th-of-Tammuz.htm

The walls of Jerusalem were breached by the Romans, in 69 CE, after a lengthy siege. (Three weeks later, after the Jews put up a valiant struggle, the Romans destroyed the second Holy Temple on the 9th of Av.)

The Jerusalem Talmud maintains that this is also the date when the Babylonians breached the walls of Jerusalem on their way to destroying the first Temple.

The sages explain: “Every generation for which the Temple is not rebuilt, it is as though the Temple was destroyed for that generation.” A fast day is not only a sad day, but an opportune day. It’s a day when we are empowered to fix the cause of that destruction, so that our long exile will be ended and we will find ourselves living in messianic times; may that be very soon.