The tribes could marry one another
The restriction on marrying between tribes in the desert generation was lifted.
Holidays15 Av · A day of joy
Six days after Tisha B'Av: the daughters of Jerusalem dancing in white, and one of the happiest days of the year.
"There were no days as joyful for Israel as the fifteenth of Av and Yom Kippur. On them the daughters of Jerusalem went out in borrowed white garments, so as not to embarrass those who had none, and danced in the vineyards" (Mishnah Ta'anit 4:8).
Coming six days after Tisha B'Av, it turns the summer from mourning to joy, and begins the season of preparing for the Days of Awe.
In the Talmud
The Talmud (Ta'anit 30b–31a) gives several reasons.
The restriction on marrying between tribes in the desert generation was lifted.
The generation decreed to die in the desert stopped dying.
The guards that kept the Northern Kingdom from going up to Jerusalem were removed.
After the fall of Betar, the Romans allowed the dead to be buried.
The last day of the year for cutting wood for the altar.
How it is marked
The day is festive, and Tahanun is omitted.
In Israel it has become a popular day for weddings, engagements and celebrations of love.
As the nights grow longer, the Sages encouraged adding Torah study at night.
Keep learning
Questions
The 15th of Av, which the Mishnah calls one of the two happiest days of the Jewish year.
No. Work is permitted; the day is marked by omitting Tahanun and by celebrations.