Tu b'Av, or the fifteenth of Av, is a minor Jewish holiday which this year falls tomorrow, on the 26th of July. It has ancient and modern origins. In the era of the 2nd Temple (Beit HaMikdash ha-Sheni) it was a day of matchmaking, for single women to be married. This was before the destruction of the Temple in 70 C.E.
After the fall of the Temple, the holiday drifted into obscurity and went unobserved for centuries. Now, with the resurrection of the State of Israel, it has popped back into popularity and is now sort of the Jewish Valentine's Day. It is seen as chag ha-ahavah, or the "holiday of love."
During the days of the Beit HaMikdash, there were religious customs associated with it. For one thing, the Talmud and Mishna state that it was a joyous holiday, where the young unmarried women would go out into the fields dressed in white (so as not to distinguish between rich and poor), and dance and sing in the vineyards. The young unmarried men would go out to watch them, and choose from among them wives for themselves.
These days, it is considered an auspicious day to be married, and although there are no specific customs which continued on to this day, it can be and is considered by some the emotional and spiritual 'end' to the three weeks of mourning which culminated in Tisha b'Av, the day we mourned the destruction of both the first and second Temples, and Jerusalem, so many centuries ago. It is the happiest day in the unhappy month of Av.
here and here.