The Fast of the 17th of Tammuz on Sunday marks the beginning of the Three Weeks of mourning over the siege of Jerusalem and destruction of the Holy Temples. It is one of four fast days connected with that tragedy and was the day that the walls of the city were breached during Second Temple times (they were breached on the 9th during the First Temple period).
This year the 17th of Tammuz occurred on Saturday, but the fast day was delayed one day because it is forbidden to fast on the Sabbath, except on Yom Kippur.
The three weeks end on the Ninth of Av, the day the First and Second Temples were destroyed. Most authorities date the destruction of the Temple to 586 B.C.E.
The 17th of Tammuz is a day on which five catastrophes befell the Jewish people
This is also the date on which Moses, having descended from Mount Sinai for the first time, saw the people celebrating the Golden Calf, and broke the first set of Ten Commandments.
In addition, the priests of the First Temple era were forced on this day, a year before the Temple's destruction, to stop offering the daily sacrifice due to the shortage of sheep.
The Talmud also teaches that the evil King Menasheh had an idol placed in the Temple's Holy Sanctuary on Tammuz 17. Later, during Second Temple times, a Roman general placed an idol in the same place and publicly burned the Torah.
The fast begins before sunrise and ends after the evening prayers, around 8:10 p.m. in Israel, depending on one’s location.
During the next three weeks, it is prohibited to hold weddings, parties, shave or have haircuts, listen to music, purchase or wear new clothes.
Mourning customs are more stringent from the beginning of the month of Av for Ashkenazim and from the week of the 9th of Av for Sephardim. During that time, doing laundry, bathing in hot water (except before Shabbat and ritual immersion), swimming, consumption of wine and meat (except on the Sabbath or at Brit Mila) are not allowed. People do not enter new business contracts, engage in disputes with a non-Jew during that period or do anything that could pose a danger, as these Nine Days are considered a non-propitious time for Jews.
Torah sages teach that the purpose of maintaining customs of mourning is not to remember the hardships suffered in the past but, as the Maimonides (Rambam) explains, to “awaken [our] hearts and clear the paths to repentance.”