Chapter 6: The Four Fasts Commemorating the Destruction of the Temple
On the third of Tishrei, Gedalia b. Aĥikam was killed. This extinguished the last ember of Jewish sovereignty in Eretz Yisrael and realized the full measure of exile. This is why the prophets instituted a fast day.
Some days possess special intensity. Such days, if we are worthy, turn into holidays. If, Heaven forbid, we are not worthy, they can become days of extreme trial - as in the time of Gedaliah ben Achikam. But the possibility of rectification remains.