The Gemara then discusses the tragedy of the wall of Jerusalem being breached, the event which led to the overrunning of Jerusalem by our oppressors and the eventual destruction of the Temples.
The Gemara notes that there is an inconsistency between our Mishnah and the verse in Yermiyahu 52:6, which implies that the wall was breached on the ninth of Tammuz, not the seventeenth as the Mishnah says.
The Gemara resolves this inconsistency by explaining that the verse in Yirmiyahu is referring to the time of the first Temple, while our Mishnah is referring to the time of the second Temple. If that is the case, why do we fast only on the 17th of Tammuz, and not on the ninth as well?
The Talmud Yerushalmi (Jerusalem Talmud) explains that in reality, the breach of the walls occurred on the 17th during both eras. However, in the time of the first Temple, because of the stress and upheaval of the time, the people became confused and miscalculated the days in the calendar. Therefore, they thought the breach occurred on the ninth. Yirmiyahu, when recording the event, wrote it down according to the erroneous calculation of the people, which was the prevalent belief as to the date of occurrence.
The Tur in Shulchan Aruch Orach Chayim 549 writes that the breaches did occur on different dates. However, because having two fasts in such close proximity would be a burden on the community, only one fast day was established. The fast day was established on the day the wall was breached at the time of the second Temple because the tragedy was greater regarding its impact on us: the exile that began at that time is the exile we currently live in. (Torah.org)
[Since the 17th of Tammuz fell on Shabbat, the fast was postponed until today, Sunday July 24th.]
