"Tisha B'Av, the Fast of the Ninth of Av, is a day of mourning to commemorate the many tragedies that have befallen the Jewish people, many of which coincidentally have occurred on the ninth of Av." (Judaism 101)
The expulsion of the Jews from Spain in 1492 happened to fall on the Ninth of Av.
My friend, Berger Boy, learned this mnemonic as a lad:
On Tisha B'Av
The Ninth of Av
Two temples were destroyed
One by Nebuchadnezzar of BabylonThe other by Titus of Rome.