We are obliged to fast, etc. This obligation derives from the words of the prophets, as it is written in Scripture (Zech. 8:19), "The fast of the fourth, the fast of the fifth, the fast of the seventh, and the fast of the tenth..." Our Sages, may their memories be for a blessing, explained that "the fast of the fourth" is the 17th of Tammuz, since it is in the fourth month according to the way we count the months; that "the fast of the fifth" is the Tish'ah B'Av [the 9th of Av], since it is in the fifth month; that "the fast of the seventh" is the Fast of Gedalia, since it is in the seventh month [Tishrei]; and that "the fast of the tenth" is the 10th of Tevet, since it is in the tenth month. On all of these days, all Israel fasts because of the misfortunes that befell them on these days, in order to arouse their hearts to open up to the paths of repentance [teshuvah]. This [fasting] will be a recollection of our evil deeds and the deeds of our ancestors which were like our deeds now, such that they caused them--and us--these misfortunes, and in the recollection of these things we will return [nashuv] to become better [people], as it is written (Lev. 26:40), "And they will confess their iniquity and the iniquity of their ancestors..." Therefore, each person is obliged to take to heart on these days to examine their deeds and to repent [lashuv] of them, because the fast [itself] is not the essential [thing], as it is written [in Jonah 3:10] about the people of Nineveh: "And the Eternal saw their deeds, that they repented [turned back] from their evil path..." The Sages, may their memories be a blessing, said that their sackcloth and their fasting were not mentioned--just their deeds. Fasting is nothing but a preparation for repentance. Therefore, those people who, as they are fasting, go on walks or [pursue] trivial matters grasp the issue of secondary importance, but loses track of the essence. But in any case, one may not exempt oneself by means of repentance alone, because to fast on these days is a positive commandment from the prophets, as we wrote above.

On account of the terrible things, etc. Because on 17 Tammuz five misfortunes occurred. On it, the tablets were broken as Moses descended from the mountain, as is explained in the Torah [Ex. 32:19]; the daily offering was cancelled and was no longer sacrificed in the First Temple; [the walls of] the city [of Jerusalem] were breached [in advance] of the destruction of the Second Temple; the evil Apostomos burnt the Torah; and an idol was placed in the Holy Temple. On on 9 Av, the great destruction occurred. The First Temple was destroyed on [that day], as was the Second; and on that very day it was decreed that our ancestors that were in the desert would not enter the land; and on that very day, [Romans] conquered a great town, called Beytar [during the Bar Kokhba revolt], which was home to thousands of Jews, all of whom fell into the hands of the idolaters and all of whom perished, which was a calamity as great as the destruction of the Temple; and on this very day, destined for calamity, the evil Turnusrufus plowed the Temple and its environs, thus fulfilling what was said: "Zion will be plowed as a field" (Jer. 26:18). And on the day of 3 Tishrei, Gedalia son of Ahikam was murdered. He had been appointed to lead the remaining survivors after the [first] destruction [and his death led to] the extinguishing of the remaining embers of Israel, because [in the wake of it] all were exiled and thousands were killed. And on 10 Tevet, the evil king of Babylon, Nebuchadnezzar assaulted Jerusalem and laid it under the siege and distress that led to the destruction.