Tu B'Av, the fifteenth day of the month of Av, is a sort of Jewish Valentine's Day. The Talmud describes it as, together with Yom Kippur, the most joyful day in the Jewish calendar - a provocative pairing (Ta'anit 30b). On Tu b'Av the maidens of Israel went out to the fields to dance and find a match. This prototype was the source for the Tu b'Av singles events replete with romantic dinners, parties, scattered rose petals, heart-shaped balloons, sunsets, moonrises and soft music that abound today.
One interesting detail of the ancient Tu b'Av dance as described in the Talmud was that the girls swapped dresses beforehand. Switching clothes softens the potential edge of competitiveness in the dance, levels the advantages of wealth and hides the embarrassment of those who had no party clothes. It represents a kind of democratisation of desire, breaking down the connections between clothes, status and our sense of who we are.