The “Three Weeks” and Tisha B’Av are designated as a time of mourning over the destruction of the Holy Temple and the galut (exile). Tisha B’Av, the 9th day of the month of Av which this year falls July 29-30, is the saddest day on the Jewish calendar. It is the culmination of a three week period of time during which they mark the destruction of both the holy temples in Jerusalem. The 9th day of the month of Av in the Jewish calendar is a recurring date throughout history.

Both the Holy Temples were destroyed on this day

The First Temple was burned by the Babylonians in 423 BCE and the Second Temple fell to the Romans in 70 CE.

Later when The Bar Kochba revolt against the Romans in 133 CE ended in defeat: The Jews of Betar were killed on this day and a year later on the same date Temple Mount was plowed. Other tragedies occurred on this same date, including in 1290 the expulsion of England’s Jews, (which surprised me) and in 1492 banishment of all Jews from Spain.  

For further information see this article https://www.chabad.org/library/article_cdo/aid/144575/jewish/What-Is-Tisha-BAv.htm