The fast of Tammuz, according to Rabbi Akiva's interpretation, is the fast mentioned in the Book of Zechariah as "the fast of the fourth ". This refers to Tammuz, which is the fourth month of the Hebrew calendar. According to the Mishnah, five calamities befell the Jewish people on this day:
The Babylonian Talmud places the second and fifth tragedies in the First Temple period. The Book of Jeremiah states that the walls of Jerusalem during the First Temple were breached on the 9th of Tammuz. Accordingly, the Babylonian Talmud dates the third tragedy to the Second Temple period. However, the Jerusalem Talmud states that in both eras the walls were breached on 17th Tammuz, and that the text in Jeremiah 39 is explained by stating that the Biblical record was "distorted", apparently due to the troubled times. The Seventeenth of Tammuz occurs forty days after the Jewish holiday of Shavuot. Moses ascended Mount Sinai on Shavuot and remained there for forty days. The Children of Israel made the Golden Calf on the afternoon of the sixteenth of Tammuz when it seemed that Moses was not coming down when promised. Moses descended the next day, saw that the Israelites were violating many of the laws he had received from God, and smashed the tablets.
Customs
As a minor fast day, fasting lasts from dawn to shortly after dusk. It is customary among Ashkenazi Jews to refrain from listening to music, public entertainment, and haircuts on fast days, and on this occasion because it is also part of The Three Weeks ; other deprivations applicable to the major fasts do not apply. If the 17th of Tammuz falls on a Shabbat, the fast is instead observed the next day, the 18th of Tammuz. This last happened in 2019, and will happen again in 2022. A Torah reading, a special prayer in the Amidah, and Avinu Malkenu are added at the morning Shacharit and afternoon Mincha services. Ashkenazi congregations also read a haftarah at Mincha. Congregations also recite during Shacharit a series of Selichot reflecting the themes of the day.
The three weeks beginning with the 17th of Tammuz and ending with the Ninth of Av are known as Bein haMetzarim, or The Three Weeks. Some customs of mourning, which commemorate the destruction of Jerusalem, are observed from the start of the Three Weeks. The oldest extant reference to these days as Bein haMetzarim – which is also the first source for a special status of The Three Weeks – is found in Eikhah Rabbati 1.29. This midrash glosses Lamentations 1:3, "All pursuers overtook her between the straits." The three weeks of mourning between the 17th of Tammuz and 9th of Av is cited as a rabbinically instituted period of fasting for the "especially pious". Such fasting is observed from morning to evening, common with other rabbi-decreed fasts.