121:1 There is a positive commandment from the prophets to fast on days on which disasters befell our ancestors. The purpose of a fast is to stir our hearts and to encourage us to repent. A fast should serve as a reminder of our evil deeds because similar deeds of our ancestors caused the disasters that still plague us today. Remembering these events should cause us to repent as per Leviticus 26:40, “to acknowledge their sins and their ancestors’ sin.” Therefore, every person must take to heart on these days to evaluate his deeds and to improve them.
The main thing is not the fast but the repentance as we see regarding the people of Nineveh, “G-d saw their deeds” (Jonah 3:10). The Talmud (Taanis 15a) points out that it doesn’t say that G-d saw their sackcloth and their fasting, it says that He saw their deeds, i.e., that they had repented from their evil ways. The fast is a means to that end, so people who fast but go out on outings and fill the day with frivolous activities are completely missing the point.
121:2 These are the fast days: 3 Tishrei was the murder of Gedaliah ben Achikam, who was appointed governor over Israel by Nebuchadnezzar after the destruction of the first Temple. Because of his murder, the rest of the nation was exiled. The dead numbered in the thousands and the last embers of the nation were extinguished.