Tu b'Av is a minor Jewish festival, of sorts, and the closest thing in the Jewish year to Valentine's Day.
Tu b'Av literally means "the fifteenth of Av", as the gematria of Tu (the Hebrew letters tet=9 and vav=6) is 15. Av is a month in the Jewish calendar, normally falling around July / August in the Gregorian Calendar.
The Talmud, in Ta'anit, ascribes a number of historical events to this date:
The running theme in these events is the end of some form of restriction. Interestingly, the end of the ban on marrying Benjaminites is recorded:
"Therefore they commanded the children of Benjamin, saying "Go and lie in wait in the vineyards. And see, and, behold, if the daughters of Shiloh come out to dance in dances, then come out of the vineyards, and catch you every man his wife of the daughters of Shiloh, and go to the land of Benjamin".
The Mishna describes what used to be done on Tu b'Av in the time of the Second Temple - in other words, the period up to about the year 70. Note the similarity to the description in Judges, above.
Mishna, Ta'anit 4:8
Rabban Shimon ben Gamliel said: "Never were there any more joyous festivals in Israel than the 15th of Av and Yom Kippur, for on them the maidens of Jerusalem used to go out dressed in white garments - borrowed ones, in order not to cause shame to those who had none of their own.
... the maidens went out and danced in the vineyards, saying: Young men, look and observe well whom you are about to choose (as a wife); regard not beauty alone, but rather look to a virtuous family, for "false is grace, and vain is beauty: a woman that fears the Lord shall indeed be praised" (Proverbs 31:30-31); and it is also said: "Give her of the fruit of her hands, and let her own works praise her in her gates".
That's right. Tu b'Av, coming at the height of summer, was a massive singles' party. It seems some of the tight restrictions about separation of the sexes were suspended that day, and everyone danced and met that special someone. But after the destruction of the Temple in 70 CE, the parties stopped. All that remained of the festival of Tu b'Av were vestigial traces in the liturgy; no Tachanun is said on that day. Couples getting married on Tu b'Av are exempt from the traditional fasting on their wedding-day, so it became a popular day to get married.
It was the secular Kibbutz movement that revived Tu b'Av. Dispite a disinterest in Judaism as a religion, they were at the forefront of creating a reborn Jewish cultural identity. They dressed in white and went into the vineyards, in celebration of the ceremony of old, and as a good excuse for a party.
Nowadays, Tu b'Av has taken on a valuable role in Israeli society, and in the world Jewish community in general; it has become a Jewish Valentine's Day. After all, a day commemorating the martyrdom of a saint is sort of problematic for non-Christians, and however Hallmark-like the day has become, its religious roots are still present. So some people send cards and presents on Tu b'Av, but it's not really a big deal for most of the Jewish world, especially not the ultra-Orthodox side. However, it's becoming more popular amongst young people trying to express their Jewishness without missing out on any of the fun stuff.
Oh, one more thing. It was Tu b'Av on the 13th of August.
http://www.sacred-texts.com/jud/t04/taa09.htm for the Mishna translation, slightly modified by me
Eliahu Kitov, The book of our Heritage