Tu Be'av is the 15th of the month of Av, where the "tet" and "vav" represent 9 plus 6, ie 15.
The date is a minor Jewish festival for several reasons and is one of a number of festivals to fall around the middle of the lunar month: others with agricultural aspects include Pesach, Succot and Tu B'Shvat, while the last also falls into a class of minor festivals falling mid-month like Purim, and even Lag Ba'Omer.
It has a number of significances, of which several have a modern interpretation. The holiday was instituted in Second Temple times to mark the beginning of the grape harvest. Since Yom Kippur also marked the end of the grape harvest, the Mishnah records that on both dates unmarried girls of Jerusalem would dress in borrowed white garments and go out to dance in the vineyards [Ta'anit 4:8]. It is not far from this to associations with match-making and weddings, although this custom itself probably stems from a far more ancient one [see below]. The closeness to the date of Tisha Be'av which precedes it by 6 days is a likely factor in the pre-eminence of joyous associations with Tu Be'Av.
During this period, this was also the last day in the series of nine wood offerings by prestigious families to the Temple, open to the Levite families and others. This is most probably the origin of the bonfire customs associated with the festival, also attributed to its associations to the pagan mid-summer solstice celebrations.
Other reasons the date was preserved are rather a mixed bag [Ta'anit 30b-31a; baba Batra 121a, b; Jer. Talmud, Ta'anit 4:11, 69c] and in post-Temple Jewish tradition it was more ignored than remembered, although there are strict festival observances: the Tahanun prayer is not said on Tu Be'av and no eulogies are pronounced at funerals.