Tisha b'Av is the anniversary of the destruction of the 1st and 2nd Temples in Jerusalem (586 BCE, and 70 AD, respectively).
The 9th (Tisha, in Hebrew) of Av is associated with many other disasters that before the Jewish people, including the defeat of the Bar Kochba rebelllion (133 CE). The expulsion of Jews from England (1290 CE), and the Spanish Inquisition (1492).
Traditionally, there are no weddings and no B'nai Mitzvah between the Fast of Tammuz (17 Tammuz/27 June 2021) and Tisha b'Av. This is a day of fasting for observant Jews.