Tammuz is the fourth month of the biblical calendar year. "Tammuz" is the Babylonian name of the month. In the few times this month is referred to in Scripture, it is simply referred to as the "fourth month," as in Jeremiah 52:6: "On the ninth day of the fourth month the famine was so severe in the city that there was no food for the people of the land."
Historically, Tammuz commemorates many tragedies for the people of God. As we just read, it was during this month that the final days of siege against Jerusalem came to a close. On the ninth day of this month the walls of Jerusalem were breached. This siege ended with the destruction of the first Temple on the 9th of Av, and many of the Jews were exiled from the land.
Judaism has a long-standing tradition of fasting to remember these occurrences. This practice originated during the days of the post-exilic prophets. The prophet Zechariah made mention of "the fasts of the fourth, the fast of the fifth, the fast of the seventh and the fast of the tenth months ..." (8:19).
The fast of Tammuz occurs on the 17th day of the month. It is a food and liquid fast from sunrise to sundown. The seventeenth also marks the beginning of the Three Weeks. From the 17th of Tammuz to the 9th of Av, faithful Jews observe a period of mourning for the destruction of both Temples. Different customs such as refraining from haircuts, music, and dancing are observed as ways to decrease one's joy. All of these have the goal in mind of awakening one to repentance and returning one's life to the proper path.
Fasting is a common practice for the disciples of Yeshua. The Master himself declared, "The days will come when the bridegroom is taken away from them, and then they will fast" (Matthew 9:15).
What is the purpose of fasting? The purpose of fasting is to stir one to repentance. If one fasts, but spends the day idly without self-examination or repentance, it is of no use. Skipping meals is not an act of righteousness. One should fast with the proper intention. We must fall upon the mercy of God, the mercy that He has revealed to us in His Son Yeshua. When we confess our sins in His name, through His perfect atonement on the heavenly altar, we have the assurance that our sins have been forgiven.
Though we mourn, we also can have "joy and gladness" (Zechariah 8:19) through our redemption in the Messiah.