In the eleventh year of Zedekiah, in the fourth month (Tammuz), on the ninth of the month, the city was broken into. (Jeremiah 39:2)

On the ninth day of the fourth month, the famine in the city reached a point that no food remained for the people. Then a breach was made in the city, and all the men of war fled and went out from the city by night by the way of a gate between the two walls, by the king's garden, while the Chaldeans were around the city. And they went in the direction of the Arabah. (Jeremiah 52:6-7)
The simple meaning of the verses is that the Babylonians breached the walls of Jerusalem on the 9 th of Tammuz, not on the 17 th. Indeed, the Babylonian Talmud questions the Mishna’s determination that the walls of Jerusalem were breached on the 17th of Tammuz:
Was it on the seventeenth? It is written: “On the ninth day of the fourth month, the famine in the city reached a point that no food remained for the people” and subsequently the verse states: “Then a breach was made in the city?” (Ta’anit 28b)
The Talmud resolves the question by stating that the walls were breached on the seventeenth by the Romans, prior to the destruction of the Second Temple, while the Babylonians breached the walls on the ninth of Tammuz, as stated in Jeremiah.
However, the Jerusalem Talmud understands that prior to the destruction of the First Temple, Jerusalem’s walls were breached on the 17th day of Tammuz. The reason the verses in Jeremiah cite the ninth of the month as the day the walls were breached is that “there was a corruption of the account,” that is “because of the great distress (with the breach of the walls) the people lost track of the actual date.” [Korban haEida, Jerusalem Talmud, Ta’anit 4:5] In ancient times, calendars were not common, and it was easy for ordinary people to lose track of the date. While Jeremiah, who wrote his book with Divine inspiration, was aware that the actual date of the breach was the seventeenth of Tammuz, he wrote the date as the ninth as a way of expressing solidarity with the ordinary people.

Perceptively, Rabbi Ya’akov Schur notes that the Babylonian Talmud quotes the verse from chapter 52, while the Jerusalem Talmud chose that from chapter 39. While both verses mention the date of nine Tammuz, there is a significant difference between them. The verse in chapter 39 states explicitly that the walls were breached on the 9th of Tammuz, while the verse in chapter 52, the final chapter of Jeremiah, can be read to mean that on the 9th of the month, the famine reached its peak, and some days (eight) later, the walls were breached. According to this understanding, day the famine reached its peak was confused with the day the wall was breached, as the Jerusalem Talmud asserts “there was a corruption of the account.”

Rabbi Schur further notes that the penultimate chapter of Jeremiah ends with the comment: “thus far the words of Jeremiah’” meaning that the final chapter is a later insertion. The Babylonian Talmud quotes the later verse, written at a time when it was clear that the actual date of the breach of the walls was 9 Tammuz, and therefore understood this to be the actual date of the wall’s breach, rejecting the reading that the date referred only to the famine reaching its peak.

Indeed, Shulḥan Aruch accepts the opinion of the Babylonian Talmud:
Though the verse states that the walls were breached on the 9th of Tammuz, we fast not on the 9th but on the 17th, since the walls (of Second Temple period Jerusalem) were breached on the 17th, and the second destruction is more drastic. (Oraḥ Ḥayyim 549:2)
Magen Avraham quotes Naḥmanides’ comment that our Sages chose not to institute a fast day on the ninth of Tammuz as well in order not to unduly burden the public, and adds that “those with lofty souls fast on the ninth as well.”

Rabbi David Abudraham that the sages of the end of the First Temple period indeed established the fast day on 9 Tammuz, but following the second destruction, the fast was moved to the 17th.
It is to be noted that both verses from Jeremiah use a passive form: “the city was broken into” and “a breach was made in the city,” as opposed to stating that the Baylonians breached the wall.
Metzudat David suggests that the intention is that the defenders of Jerusalem were so weakened by the famine that they could offer no resistance to the Babylonian soldiers, and it was as if the wall were breached by itself.
Malbim offers a fascinating alternative explanation: it was not the Babylonians who broke into the city, but the starving Jerusalemites who, due to the extreme famine, broke out of the city. Thus, the Babylonian army did not breach the walls, but merely entered through the breach made by the besieged Jerusalemites.
There is great symbolic significance to Malbim’s elucidation: ultimately, the Israelites bear the primary responsibility for the breach of the walls which led to the Temple’s destruction. In truth, it was not the Babylonians who destroyed Jerusalem, but the sins of Israel. The Babylonians were merely God’s agents for executing His decree. (David Magence)

Shabbat Shalom!
The Va’ad