The rabbis counted the wounds and found that five had opened on the seventeenth of Tammuz and five more on the ninth of Av, the two fast days that frame the Three Weeks of summer mourning.
On the seventeenth of Tammuz the heart of the covenant cracked. Moses came down from Sinai, saw the golden calf, and smashed the tablets of stone (Exodus 32:19). That was the first wound. Then, during the Roman siege, the daily sacrifice in the Temple stopped for lack of lambs. The walls of Jerusalem were breached by Titus's men. A Roman officer named Apostumus burned a Torah scroll in public. And an idol was set up inside the sanctuary itself.
Three weeks later, on the ninth of Av, the wounds multiplied. It was on this date, the rabbis teach, that the spies returned with their evil report and God decreed that the generation of the wilderness would not enter Canaan (Numbers 14:29). On this date the first Temple fell to Nebuchadnezzar in 586 BCE. On this date the second Temple fell to Titus in 70 CE. On this date, fifty-two years later, the fortress of Betar fell, ending Bar Kokhba's revolt. And on this date, a year after Betar, the Roman general Turnus Rufus plowed the Temple Mount with oxen, sowing it like a field.
This Talmudic teaching from Taanit 26a-b, preserved in Hebraic Literature (1901), is why Jews fast on both days. Memory refuses to let calamity become coincidence.