Tisha B’Av, the 9th day of the month of Av, is the saddest day on the Jewish calendar, on which we fast, deprive ourselves and pray. It is the culmination of the Three Weeks, a period of time during which we mark the destruction of the Holy Temple in Jerusalem. Read more https://www.chabad.org/library/article_cdo/aid/144575/jewish/What-Is-Tisha-BAv.htm