Today, July 22, 2018, 10 Av according to the Jewish calendar, many Jews fast. The fast this year is not on 9 Av because 9 Av, Tisha B’Av in Hebrew, occurs on Shabbat when it is inappropriate to fast, so the fast is delayed until the next day.
Tradition states that the ninth of Av commemorates the destruction of both the first and second temples in Jerusalem, in 586 BCE and 70 CE, respectively. Like the fast of the 17th of Tammuz, it is not based on historical facts. We no longer know why these dates were chosen for the fasts. Interestingly, we have dates when the temples were destroyed and they are not the ninth of Av. The biblical book II Kings 15:8 states that the First Temple was destroyed on the seventh of Av, while the biblical book Jeremiah gives the tenth as the destruction date. Tosephta Ta’anit 4:6 and the Babylonian Talmud, Ta’anit 29a, try to resolve the apparent discrepancies by stating that the destruction of the First Temple by the Babylonians began on the seventh, they ignited a fire in the Temple on the ninth, and it continued to burn on the tenth. The historian Josephus, who lived when the Second Temple was destroyed and saw it burn, gives the destruction date as the 10th of Av in his book Wars of the Jews.
The Babylonian Talmud, Ta’anit 26b, states that in addition to the destruction of both temples, three other events occurred on the ninth of Av: the decree was issued during the days of Moses that the Israelites could not enter Canaan but must instead remain in the desert for 40 years, the city of Bethar was destroyed during the Bar Kochba rebellion of 132–135 CE, and the Romans razed Jerusalem. There is no evidence, historical or otherwise, that these events occurred on 9 Av. Significantly, the Talmud is telling us that both fast days 9 Av and 17 Tammuz recall five tragic events.
The fact that it was believed that the Second Temple was destroyed in the same month as the First Temple made a tremendous impression upon the Jews. In the words of Josephus, “One cannot but wonder of the accuracy of this period thereto relating; for the same month and day were now observed wherein the holy house was burnt formerly by the Babylonians.” Some sages understood this to be far more than coincidence. They believed that God ordains events so that positive developments occur on auspicious days and tragedies occur on days predestined for disaster. They go so far as to advise people not to undertake any new venture on 9 Av because the day is so unlucky that the venture will surely fail.
 The following are some paragraphs from my book “Mysteries of Judaism II.”
 Wars of the Jews 6:249–250.
 Ibid. 6:268.
 Babylonian Talmud, Ta’anit 29a.