The Fast of 17 Tammuz occurs this year on July 20 which is a Saturday, so the fast is postponed to Sunday July 21, as will be explained below. The Jewish calendar contains a mourning period during the summer months. It begins and ends with fast days, whose precise dates are not based on verifiable facts. The mourning period chiefly commemorates the destruction of both temples and serves to remind Jews of their history and their connection to the land and people of Israel.
The Three Weeks
The three-week mourning period begins on the seventeenth of the Hebrew month Tammuz and ends on the ninth of Av. The period is colloquially referred to as “the three weeks.” The seventeenth of Tammuz is called a minor fast day, meaning the fast begins in the morning rather than sundown of the prior day. The ninth of Av, on the other hand, is a major fast because the fast begin at sunset and is longer than seventeen Tammuz.
Traditions differ as to what Jews do during the three weeks. Some mourn the loss of the two temples by refraining from pleasures either during the entire three weeks, the nine days of Av, or the last three days before 9 Av. Many recall the ancient tragedies and mourn them by not eating meat dishes during the nine days of Av, because they consider meat to be a source of joy and therefore an improper consumption during mourning periods, and by not having a haircut, or men shaving. The Talmud, written in the sixth century, does not mention the three-week period, but it discusses the different behaviors of people during the nine days of Av.
The fast of 17 Tammuz is first mentioned in Zechariah 8:19. It commemorates the beginning of the siege of Jerusalem’s walls during the First Temple period, in 586 BCE. However, while Zechariah cites the month of the fast day, it does not give the date. The Babylonian Talmud, Taanit 26a–b, states that four additional tragedies happened on 17 Tammuz: Moses broke the first set of the Decalogue, Romans placed an idol in the temple’s inner sanctum during the Second Temple period, the twice-daily sacrifices offered in the temple were forcibly discontinued, and a Roman military leader burned a Torah scroll. There is no historical evidence that the onset of the siege or any of the other four events occurred on 17 Tammuz, other than Jewish tradition.
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 seventeenth 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 Taanit 4:6 and the Babylonian Talmud, Taanit 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 tenth of Av in his book Wars of the Jews.
The Babylonian Talmud, Taanit 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 forty 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 recall five tragic events.
The fact 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 fasts are not always observed on 17 Tammuz and 9 Av. This is because the rabbis felt that fasting runs counter to the spirit of the festive Sabbath. Most fasts, therefore, are postponed until Sunday; others are pushed ahead and observed on the preceding Thursday or Friday. The Mishna lists several ceremonies that are postponed, including 9 Av. Although not mentioned in this Mishna, the fast of 17 Tammuz also gets postponed, as it does this year.
It is clear from the above that Jewish ancestors instituted fasts around the time of the events they commemorate, not specifically on the exact dates in which the events occurred. This is like the holiday of Shavuot (Pentecost). It was only during the Middle Ages that the connection was made between Shavuot and the giving of the Decalogue (Ten Commandments) to the Israelites. Shavuot is called today in Hebrew zeman matan Torateinu, “the season of the giving of the Torah,” even though the Torah does not give the exact date for the revelation of the Decalogue, it is just the “season” when it was revealed.
Why Was the Date of 9 Av Chosen?
Since, as previously stated, the temples were not destroyed on 9 Av, why did the rabbis say the demolition of the first and second temples occurred on this date? We do not know. But it is possible that the rabbis didn’t want to associate the destruction of the temples with the numbers seven, eight, and ten.
It could be that the rabbis rejected seven because seven is used frequently in the Bible to denote completeness, and it reminds us of Shabbat and joy; the rabbis did not want to give the impression that Judaism was completely destroyed, nor did they want to connect the destruction to joy.
Eight may not have been chosen since eight is a symbol of a new beginning, since it follows seven; the temple destruction was not a new beginning, but the end of an era. Similarly, ten may have seemed inappropriate because ten is made up of seven and three, the two numbers used in the Bible to signify completeness.
It is also possible that the rabbis did not choose to have the fast of Av on the tenth because the tenth would remind Jews of the other fast of the tenth, Yom Kippur, and they did not want Jews to compare the two days. The fast of Av is considered a total calamity. Yom Kippur, on the other hand, has positive elements: In the times of the temple, young men and women would meet and dance on the afternoon of the holiday. Additionally, on Yom Kippur, the revamped Yom Hakippurim, one can seek forgiveness and repair past misdeeds.
 Wars of the Jews 6:249–250.
 Ibid. 6:268.
 Babylonian Talmud, Taanit 29a.
 Mishna Megilla 1:3.
 See Mysteries of Judaism I, chapters 8–9.
 Abraham ibn Ezra explains that the number seven indicates something is complete while the number three, being close to half seven, suggest a smaller completeness, such as Abraham taking a trip for three days. See my Maimonides and the Biblical Prophets (Jerusalem: Gefen Publishing House, 2009), especially page 300.
 See my Mysteries of Judaism I for the differences between the biblical Yom Hakippurim and Yom Kippur.