Shabbat during the Three Weeks

The three weeks between the 17th of Tammuz and Tisha B'Av are a time of sadness and mourning over the destruction of the Holy Temples. On Shabbat, however, we are forbidden to mourn. Even if Tisha B'Av falls on Shabbat the fast is postponed until the next day, and the day is spent as a day of joy. These Sabbaths are like the "cure which is sent before the illness," the illness being the destruction which was to follow. These Sabbaths also reveal to us the inner meaning of the exile, which is only a preparation for the final redemption, described as "a day which is entirely Shabbat." When Moshiach comes and everything is revealed, the Jewish people will even thank G-d for their years in exile. (Lubavitcher Rebbe)