Tu B'Av was a festive day when all the Jewish women would be permitted to go out and they all would wear the same white clothing (so no woman shamed another) and they would be permitted to meet the Jewish men publicly. It does not promote illicit relationships rather it promotes marriage between Jews of different families.
Here is a relation of the story of Tu B'Av from the Temple Institute:
http://www.templeinstitute.org/tu-bav-5771.htmTu b'Av: The Happiest Day of the Year!From the Babylonian Talmud, tractate Ta'anit 30b-31a:
"There were no holidays so joyous for Israel as the Fifteenth of Av (Tu b'Av) and Yom HaKippurim, for on those days, daughters of Jerusalem 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 Israel 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."
"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...' (Proverbs 31:30)"
Young girls dancing in Jerusalem on the Mount of Olives facing the Holy Temple. Tractate Ta'anit goes on to list five different reasons for rejoicing on Tu b'Av:
During their forty year desert, female orphans among the Israelites who had no brothers could marry only within their tribe, in order to prevent their father's inherited land in the Land of Israel from passing on to other tribes. On the fifteenth of Av of the fortieth year, this ban was lifted. This we learn from the story of the five daughters of Tzelaphchad
Also during the fortieth year in the desert, the last of the generation of the sin of the spies, which had been forbidden to enter the Promised Land, found that they were not destined to die. For forty years, every Tisha B'av night, the Jews made graves for themselves which they slept on Tisha B'av; every year a number of them died. In the 40th year, the fifteen thousand remaining survivors from the generation of he spies went to sleep in their graves and all woke up the next day. Thinking they made a mistake calculaing the date, they returned each night to their graves, until the night of Tu B'Av. This being the full moon, they understood that they had not been mistaken: The decree against the generation had been rescinded!
The Tribe of Benjamin was allowed to intermarry with the other tribes. (A ban had been enacted against intermarriage following the incident of the Concubine of Gibeah, see Judges 19-21.)
Cutting of the wood for the main altar in the Temple was completed for the year: One of the early signs of the changing seasons was an increase in humidity, creating conditions in which the wood collected after the fifteenth could be wet or wormy.
More than a year after the destruction of the Jewish stronghold of Beitar and the slaughter of the entire Jewish population of the city by the Roman invaders, Rome at last permitted burial of the victims. Miraculously, the bodies had not decomposed, despite their being left unattended for the entire year. This was a sign of G-d's mercy.
During the time of the second Holy Temple, the entire seven day period, beginning with the 9th of Av an concluding with Tu b'Av, was a festive holiday celebrated in the holy Temple. When the Temple is rebuild and the Divine service renewed, so too will the seven day festival of Av be once again celebrated by Israel!
Young girls dancing in the vineyards of Shilo, where the Tabernacle dwelled for many centuries.