On the 15th of the summer month of Av, under a full moon, young Jewish men and women dressed in white would go out and dance in the vineyards of ancient Judea.Most of the explanations don't sound very romantic.
That is practically all we know about this most ancient of holidays.
We know it because of a single passage in the Mishnah, quoting Simeon ben Gamliel: “No days were as good for Israel as the 15th of Av and the Day of Atonement, on which the sons of Jerusalem would go out in borrowed white clothes...and the girls of Jerusalem would go out and dance in the vineyards” (Ta’anit 10).
The question is why. There are a few clues.
There's something about marriage
What was this dancing about? The sages of the Talmud were evidently somewhat puzzled by this, since the Talmud gives us six different answers.
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The Jerusalem Post has coverage as well: Tu Be’av: Boy meets girl... in ancient Israel (Stephen Gabriel Rosenberg)