Shulchan Arukh Orach Chayim · Siman 549

English

We are required to fast on the 9th of Av, [and] on the 17th of Tammuz, and on the 3rd of Tishrei, and on the 10th of Teves because of the bad occurrences that occurred on those [days].

English

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).