The 9th of Av, Tisha b'Av, commemorates a list of catastrophes so severe it's clearly a day specially cursed by G‑d. Another confirmation of our deeply held conviction that history isn't haphazard.
As part of our mourning for the destruction of the Temple and the exile of Israel, we abstain from many pleasurable activities on the night and day of Av 9, first and foremost: eating and drinking.