Tisha B’Av is a holiday painfully observed. For on this holiday one remembers the destruction of the Temple(s) that occurred on this date along with other tragedies to the Jewish people and the world (see link below). They say history repeats itself if we don’t learn the lessons of it Tisha B’Av makes sure we never forget them.
https://www.chabad.org/library/article_cdo/aid/946703/jewish/What-Happened-on-the-Ninth-of-Av.htm