Halakhah do Jeremiasza 39:2

It is written in the final chapter of Taanit‏ ‏‎(4:6), “Five events happened to our‎‏ ‏ancestors on the ‎seventeenth of Tammuz, and five on the ninth of Av. Onthe seventeenth of Tammuz the tablets‏ ‏were broken [by Moshe Rabbeinu upon‏ ‏seeing the golden calf], the Tamid [twice-daily] offering ‎ceased, the city‏ ‏‎[Jerusalem] was breached, Apostomus‎‏ ‏burned the Torah and an idol was‏ ‏erected ‎in the sanctuary. On the ninth‏ ‏of Av, it was decreed that our ancestors‏ ‏would not enter the land ‎‎[of Israel, due‎‏ ‏to the sin of the spies], the first and‏ ‏second Temples were destroyed, Beitar‏ ‏was ‎captured, and the city [Jerusalem]‎‏ ‏was plowed over, [specifically regarding‏ ‏when] Turnus Rufus ‎plowed over the‏ ‏sanctuary.”‎ Rabbi Akiva explained (Rosh HaShanah‏ ‏‎18b), “Thus said Hashem: the fast of the‎‏ ‏fourth and the fast ‎of the fifth and the‏ ‏fast of the seventh and the fast of the‏ ‏tenth will become days of joy and ‎rejoice‏ ‏for the house of Judah.” (Zechariah‏ ‏‎8:19) The fast of the fourth is the‎‏ ‏seventeenth of ‎Tammuz, upon which the‏ ‏city was breached, as it is written, “In‏ ‏the fourth month, on the ninth of ‎the‏ ‏month, the famine in the city become‎‏ ‏stronger,” and it is then written, “And‏ ‏the city was ‎breached”. (Yirmiyahu‏ ‏‎52:6‎‏ ‏‎-7) And why is it the fast of the fourth?‎‏ ‏Because it is in the fourth ‎month, as we‏ ‏count the months from Nisan‏ ‏‎…‎‏ ‏Even though the text (Yirmiyahu 39:2)‎‏ ‏says ‎regarding the fast of the fourth‏ ‏month, “On the ninth of the month the‏ ‏city was breached,” now ‎we fast on the‏ ‏seventeenth of the month. Initially the‏ ‏fast was established on the ninth of the‏ ‏month, since the ninth is the day the‏ ‏city was breached in the first [Temple].‎‏ ‏However, in the ‎second [Temple] the city‏ ‏was breached on the seventeenth, and‏ ‏so they established [the fast] on ‎the‏ ‏seventeenth. This is because the‏ ‏destruction of the second Temple is‏ ‏harsher for us. In any ‎event, the fast of‏ ‏the fourth is due to the breaching of the‏ ‏city, and for the pains that were‏ ‏multiplied on that day.‎

Even though it is written in the verse (Jeremiah 39:2) "[During the eleventh year [of the reign] of Tzidkiyahu] in the fourth month [i.e., Tammuz], on the ninth day of the month, the city was crossed", we do not fast on the 9th day of that [month], rather on the 17th of that [month] -- since, despite the fact that originally (i.e.; the First Temple) [the city of Jerusalem] was breached on the 9th of that [month] -- since the second time (i.e.; the Second Temple) [the city] was breached on the 17th of that [month], [the sages] decreed a fast on the 17th of that [month] because the destruction of the Second Temple was stronger than it (i.e; the destruction of the First Temple).