The observance of the holiday is outlined the last mishna in masechet (tractate) Ta'anit (the following translation is quoted from the OU):
"There were no holidays so joyous for the Jewish People as the Fifteenth of Av and Yom HaKippurim, for on those days, daughters of Yerushalayim would go out dressed in borrowed white clothing (so that they would all look the same).
The King's daughters would borrow from those of the High Priest. Daughters of the High Priest would borrow from the Assistant High Priest's daughters; daughters of the Assistant would borrow from the daughters of the Priest designated to lead the People in times of War, the Kohen Anointed for War's daughters would borrow from the daughters of the Ordinary Priest. And the daughters of the rest of the Jewish People would borrow from each other, so as not to embarrass those who didn't have."
"And the daughters of Jerusalem would go out and dance in the vineyards located on the outskirts of the city. And everyone who didn't have a wife would go there." (Notice the relative lack of concern about controlling the situation when the opposite sexes are mixed, perhaps because the recent fast (in the case of Tu B'Av) and the fast on that very day in the case of Yom Kippur, have triggered a sense of self-control, which would not ordinarily necessarily be present.)
"And what would they say?"
"Young man, lift up your eyes and choose wisely. Don't look only at physical beauty - look rather at the family - 'For charm is false, and beauty is vanity. A G-d - fearing woman is the one to be praised...' ("Mishlei"/Proverbs 31:30)"
While today most communities do not observe Tu B'Av as it was observed in Mishnaic times, it is still considered a prime day for finding one's bashert (intended).