In Jewish tradition, Moses shattered the tablets on the 17th of Tammuz, after he came down from Sinai and found the people worshiping the Golden Calf. This tragedy was seen as prophetic, since it was on this same date that the walls of Jerusalem were breached by the invading armies of Nebuchadnezzar (see 2 Kings 25:2-7), an event which led to the destruction of the Temple and the exile of the people three weeks later, that is, on the ninth of Av. Today, the 17th of Tammuz is commemorated as a fast day (i.e., the "Fast of Tammuz") that marks the beginning of a three week period of mourning for the loss of the Temple in Jerusalem, called the "Three Weeks of Sorrow." This three week period is also called yemei bein ha-metzarim (יְמֵי בֵּין הַמְּצָרִים), "the days in the midst of distress," a phrase taken directly from the Book of Lamentations: "Judah has gone into exile, her pursuers have all overtaken her in the midst of her distresses" (בֵּין הַמְּצָרִים).
Spiritually, the three weeks are marked by a renewed called for teshuvah (repentance), and the weekly readings from the prophets all warn the people about imminent judgment from heaven. Because this is such a somber time for the Jewish people, it is customary not to schedule weddings or other joyous events during this time of year. Indeed, among the very Orthodox, the last nine days of the three weeks are the most rigorous and solemn. Beginning on the first day of the month of Av, traditional mourning customs are practiced in anticipation of the great fast day of Tishah B'Av, when the Book of Lamentations (Megillat Eichah) is plaintively recited during the evening service.
The Three Weeks of Sorrow (culminating in Tishah B'Av) undoubtedly marks the saddest and most solemn time of Jewish year, with great emphasis placed on the need for repentance and heartfelt cries for the salvation of the Jewish people. It is a time for us to remember Israel all the more in our prayers and ask for God's revelation of the Messiah.