Asara B'Tevet , marks the beginning of the siege of Yerushalayim, during the reign of Nebuchadnezzar, that resulted in the destruction of the First Bait Hamikdash. Today, Asara B'Tevet has an added meaning. On this date, Kaddish is recited for these souls. Shiva Asar B'Tamuz, the date when the walls of Yerushalayim were breached, and other tragic events in Jewish history occured. The saddest day in the Jewish Calendar, recalls the destruction of the First and Second Temples.
Later traditions connect the Fast of Av with subsequent tragic events in Jewish history.
The Fast of Esther gives us a different perspective on fasting. Sure, it commemorates a terrible day in Jewish history, but it is not a sad day in of itself. In this case, the fast adds to the joy of Purim and makes us even more aware of Hashem s hand in the destruction of Haman. The Fast of Esther, which begins on the morning of Adar 13, commemorates the three days that Esther fasted before approaching King Achashverosh on behalf of the Jewish people. It is also the anniversary of the day that the Jews fought against their enemies throughout the Persian empire.