Tisha B'Av תשעה באב
The fast and day of mourning on which both Temples fell and nearly every great catastrophe struck.
The Mishna lists five calamities that fell on the ninth of Av. The first is the verdict on the desert generation after the spies: the people wept for nothing, and, say the sages, «you wept without cause — I shall establish for you a weeping for the generations». Then the destruction of the First Temple in 3338, of the Second in 3828, the fall of Betar in 3892, and the ploughing of the Temple Mount.
Later dates gathered to the same day: the expulsion from England, the expulsion from Spain in 5252, the outbreak of the First World War.
The fast lasts a full day, as on Yom Kippur, with mourning customs: sitting low, no leather shoes, no greetings. Eicha is read by candlelight. The Talmud blames the Second Temple's fall not on Rome's strength but on baseless hatred.