The Ninth of Av is not specifically a biblical holy day. Rather, it commemorates a number of disasters that happened to the Jewish people, traditionally all on that same day of the year. These include the destruction of the Jerusalem Temple by the Babylonians, the destruction of the Herodian Temple by the Romans, and the fall of Betar during the Bar Kokhba revolt.
Last year's Tisha B'Av post is here, leading to many links. More recent posts are here and here.
UPDATE (10 August): See also here and here.
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