The "Three Weeks" of gradually-increasing mourning over the destruction of the Holy Temples and Israel's exile begins Thursday with the fast of the 17th of the Hebrew month of Tammuz. This is the day on which Nebuchadnezzar and his Babylonian forces breached the walls of Jerusalem, after many months of siege, on their way to destroying the Holy Temple.
The Three Weeks end on the 9th of Av - Tisha B'Av - the date on which both the First and Second Temples were destroyed, roughly 2,500 and 2,000 years ago, respectively.
The 17th of Tammuz marks other calamities in Jewish history as well. It is the date on which Moses, having descended from Mount Sinai and seeing the people sinning with the Golden Calf, 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 on this date some decades earlier, the evil King Menasheh had an idol placed in the Temple's Holy Sanctuary. Later, during Second Temple times, a Roman general placed an idol in the same place and publicly burned the Torah.
The fast ends at 8:10 PM, but other mourning customs, such as no weddings or haircuts, continue until the morning after Tisha B'Av.
Like other fast days, the morning prayers include special selichot prayers, mourning our losses and asking for forgiveness. Excerpts from the selichot of the 17th of Tammuz:
"We rebelled against Him Who dwells in heaven, therefore we were scattered in all directions... We acted rebelliously before Thee with slandering tongues, therefore our tongues were made to learn to utter lamentation... The tempest-tossed afflicted people were utterly broken up and dispersed; the dry land became a boat wrecked for lack of a captain; she received [punishment] for her sins with principal and double interest, with mourning and moaning... Their adversaries assailed them on that day and... drove the nation like a chased gazelle, and there was none that sought to protect it... Turn to us, O Thou that dwellest on high, gather our dispersed from the four corners of the earth. Say to Zion, Arise! And we shall arise. Convert the 17th of Tammuz into a day of salvation and comfort." (translation by Rev. Abraham Isaac Jacob Rosenfeld)
Click here to read articles on the Three Weeks.