The title of this post may sound something like, “What color is George Washington’s white horse?” but things are not so simple.
תניא אמר ר"ש ארבעה דברים היה ר"ע דורש ואני אין דורש כמותו צום הרביעי זה ט' בתמוז שבו הובקעה העיר שנאמר (ירמיהו נב) [בחודש הרביעי] בתשעה לחדש ויחזק הרעב בעיר ולא היה לחם לעם הארץ ותבקע העיר
One of the events which we commemorate in our fast is the breach of the wall of Yerushalayim. Yirmiyahu haNavi tells us that the breach did not occur on 17 Tamuz, but rather on 9 Tamuz. So why are we fasting on the 17th? Why not hold the fast of 17 Tamuz on the 9th?
There are two basic answers to this question:
1) The Rishonim (Tos R”H 18b, Ramban) explain that the breach of the city walls during churban bayis sheni occurred on the 17th of the month. Since that churban was more severe than the first, we fast on the 17th.
2) The Yerushalmi answers that in the confusion and panic leading up to the churban there was a mix up as to the dates. The city walls were breached on the 17th, but the people thought it was only the 9th.
It could be that these are simply two equally viable alternatives, but it could also be that there is a point of machlokes underlying the answers. Perhaps the Yerushalmi was not convinced that the second churban was more severe than the first, or, if it was (a more plausible assumption so as to avoid making a machlokes in metziyus), that is perhaps insufficient license to warrant changing the date of the fast. It seems from the language of the Rishonim that this latter point bothered them. In explaining why we fast on the 17th instead of the date of the 9th that the Navi refers to, Ramban adds a justification for moving the date – since the point of the original fast was to commemorate the breach of the city walls, if that event historically moved to a different date during the second churban, the fast should move as well. According to Ramban the reason for the fast defines its date, its parameters. It could be that this is the sticking point for the Yerushalmi. Whenever one speaks of a takanah derabbanan, the reason for the law’s institution does not always match or define its parameters. This is why, for example, even when the reason behind a takanah is no longer relevant, in many cases the law remains in effect (as GR”A assumes in many places). One could argue that even if the date of 9 Tamuz no longer makes sense, if the ta’anis was instituted on 9 Av, that date should remain binding. Therefore, the Yerushalmi learns that even during churban bayis rishon, the true date of the fast was 17 Tamuz.