Tisha B'Av is an annual fast day in Judaism, on which four disasters in Jewish history are supposed to have occurred. The two primary ones are the destruction of Solomon's Temple by the Neo-Babylonian Empire in 586 BCE and the Second Temple by the Roman Empire in 70 CE. The third one is the Fall of Betar, Bar Kochba's last stronghold during the Third Jewish War against Rome (132-135 CE), and the fourth is the expulsion from Spain in 1492.
Tisha B'Av is regarded as the saddest day in the Jewish calendar. Tisha B'Av falls in July or August in the Gregorian calendar. The observance of this day includes five prohibitions, most notable of which is a day-long fast. The Book of Lamentations, which mourns the destruction of Jerusalem, is read in the synagogue, followed by the recitation of kinnot. The latter are liturgical dirges that lament the loss of the Temples and Jerusalem.
In 2021, the year 5781 in the Jewish calendar, Tisha B'Av (the 9th day of the Jewish month of Av), starts on the evening of Saturday, July 17 and ends on the evening of Sunday, July 18.
The following are some online resources:
From My Jewish Learning, Tisha B'Av 2019
Guide for Tisha B'Av from Hebrew Union College