Tisha B’av
Tisha B’av is considered the saddest day of the year.
A full-day fast during the summer commemorating disasters experienced by the Jewish people throughout history, most notably, the destructions of the Temples in Jerusalem.
Aside from Yom Kippur, the only other full-day fast on the annual calendar is the fast of the Ninth of Av, popularly called by its Hebrew name, Tishah Be’av (also spelled Tisha B’av).