Tisha B'Av is a Jewish fast day commemorating several tragedies the Jewish people have endured, including the destruction of the first and second temples. Av is the fifth month of the Jewish calendar, and Tisha B'Av means “the Ninth of Av.” The day falls in July or August of the Gregorian calendar. Since the first two temples were destroyed on the same calendar day (Av 9), tradition has assigned a gloom to this day—some see it as a day cursed by God because of Israel’s national sins.
Tisha B'Av is the final, climactic day of a 21-day period of increasing mourning called the Three Weeks. The Three Weeks is also called Bein HaMetzarim, or “between the straits,” because Lamentations 1:3 says, “Judah is gone into captivity because of affliction, and because of great servitude: she dwelleth among the heathen, she findeth no rest: all her persecutors overtook her between the straits” (KJV, emphasis added).
The mourning period leading up to Tisha B'Av begins in the previous month, Tammuz 17, a day that commemorates the first breach of Jerusalem’s walls by the Babylonians before they destroyed the first temple. During the Three Weeks, observant Jews refrain from holding public celebrations. No weddings are scheduled during the three weeks leading up to Tisha B'Av. The focus is on mourning and repentance. The final nine days, starting with Av 1, require increased austerity: no wearing of new clothes, no eating of pleasurable foods, and no bathing beyond what is essential.
On the day of Tisha B'Av itself, Jews keep a total fast, sit on the floor, recite prayers of mourning, and read the book of Lamentations. An exception is made when Av 9 falls on the Sabbath—in that case, the fasting and mourning are observed on Av 10.
Over the years the meaning of Tisha B'Av has broadened into a remembrance of Jewish tragedies throughout history, but it remains primarily focused on the destruction of the two temples.
Following Tisha B'Av, the fast is broken, but some of the other restrictions associated with mourning continue until Av 10. Then begin the Seven Weeks of Comfort, which continue through the rest of Av and the month of Elul. During this period the focus in the synagogues turns to the glorious future God has promised Israel.
The observance of Tisha B'Av is not commanded in the Bible. Like Purim, Tisha B'Av is a traditional observance based on non-canonical Jewish writings and oral tradition. It’s possible that a Tisha B'Av observance is alluded to in the book of Zechariah. The men of Bethel sent a delegation to the prophets in Jerusalem asking, “Should I mourn and fast in the fifth month, as I have done for so many years?” (Zechariah 7:3). The fifth month is, of course, Av; the “fast” mentioned could have been observed on Av 9. God’s response to the people’s question is key: “When you fasted and mourned in the fifth and seventh months for the past seventy years, was it really for me that you fasted?” (verse 5). As with any religious observance, God is more concerned with one’s motivation and the condition of the heart than He is with the ritual itself.