What does each fast day commemorate?

Tzom Gedaliah (the Fast of Gedaliah) commemorates the killing of the Jewish governor, Gedaliah, after the first exile in 586 B.C.E. As a result, the remaining Jews in Eretz Israel were all sent into exile as well. Yom Kippur is the day upon which the entire world's fate is decreed for the coming year. 10 Tevet was the day upon which Jerusalem was besieged by Nebuchadnezzar, prior to the fall of the First Temple. It is also the day proclaimed by the Israeli Chief Rabbinate as Yom Ha-Kaddish, the memorial day for the six million Jews murdered in Europe by the Nazis. Ta'anit Esther (the Fast of Esther) commemorates the three days that Esther fasted before daring to enter King Ahasuerus' private chambers uninvited. 17 Tammuz is the day that the walls of Jerusalem were breached by Nebuchadnezzar, leading ultimately to the destruction of the First Temple in 586 B.C.E. 9 Av is the day that both the First and Second Temples were destroyed (in 586 B.C.E. and 70 C.E. respectively).

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