Tubav, Hebrew Valentine's Day
The 15th of Av, sometimes associated with romantic love and jokingly called 'Jewish Valentine's Day'.
Tu B'Av (Hebrew: ט"ו באב, the fifteenth of the month Av) is a minor Jewish holiday. In modern-day Israel, it is celebrated as a holiday of love (Hebrew: חג האהבה, Hag HaAhava), similar to Valentine's Day.It is considered a very desirable date for Jewish weddings.
According to the Talmud, Tu B'Av was a joyous holiday in the days of the Temple in Jerusalem, marking the beginning of the grape harvest. Yom Kippur marked the end of the grape harvest. On both dates, the unmarried girls of Jerusalem would dress in white garments and go out to dance in the vineyards [Ta'anit 4]. (Babylonian Talmud, tractate Ta'anit 30b-31a) 
The Talmud states that there were no holy days as happy for the Jews as Tu B'Av and Yom Kippur. The holiday celebrated the wood-offering brought in the temple (see Nehemiah 10:35). Josephus refers to it as the Feast of Xylophory ("Wood-bearing").
Various reasons for celebrating on Tu B'Av are cited by the Talmud and Talmudic commentators:
* While the Jews wandered in the desert for forty years, female orphans without brothers could only marry within their tribe, 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. * That same year, the last of the generation of the sin of the spies, which had been forbidden to enter the Promised Land, died out.
* The Tribe of Benjamin was allowed to intermarry with the other tribes after the incident of the Concubine of Gibeah (see Judges chapters 19-21).
* Cutting of the wood for the main altar in the Temple was completed for the year.
* The nights, traditionally the ideal time for Torah study, are lengthened again after the summer solstice, permitting more study.
* The Roman occupiers permitted burial of the victims of the massacre at Bethar. Miraculously, the bodies had not decomposed, despite exposure to the elements for over a year.
Although the day has no specific observances in modern times, it is considered an auspicious day for marriage. It also marks an informal "high" to counter the "low" of the The Three Weeks leading up to Tisha B'Av. It is considered in modern times to be a sort of Jewish Valentine's Day, an optimal day for weddings, proposals, and romantic dates in Israel. References
1. ^ U.S. vigils remember slain gay Israelis, Jewish Telegraphic Agency, August 4, 2009
2. ^ Tu B'Av: Reclaiming old traditions, Yedioth Ahronoth, Yoav Friedman, August 4, 2009
3. ^ White dresses that are just right for Tu Be'av, Jerusalem Post, Greer Fay Cashman, August 10, 1995
4. ^ About Tu Be'av
5. ^ Bellum Judaisum 2:17
6. ^ Mishna Taanit 4:8 and Babylonian Talmud 30b and 31a, Rashi on these