There are two days on the Jewish calendar that were considered the most joyous among the Jews of the Temple period: Tu B'Av and Yom Kippur (Talmud Bavli Taanit 30b-31a). The two holidays actually have a number of themes in common, but clearly Yom Kippur as the Biblical Day of Atonement is much better known.
Tu B'Av was the day during the times of the Temple that the Jews stopped cutting wood for the Temple each year, which was a day of great rejoicing since they could rest from this particular labor. In this way it is connected to Yom Kippur since Yom Kippur was the day on which the most elaborate Temple rituals took place. It was on both these days as well the daughters of Israel would go out in borrowed white clothing in order to meet prospective husbands. Because of this, modern Israelis observe it as a Jewish "Valentine's Day", but this is not how traditional Jews understand it. Instead, it is observed now by just not reciting Tahanun, the prayer for forgiveness.
The theme of forgiveness of sin is also tied up into both days, certainly something joyous as well. On Yom Kippur, G-d designated a special day for the Jews to repent and extra rituals by which to accrue extra merit to help with the repentance issue. Tu B'Av however was a day originally initiated by the Jews themselves to thank G-d for the end of the 40 years in the desert where Jews were dying every Tisha B'Av, six days earlier. Also, marriage itself is connected to the idea of atonement since a person's sins are erased as a result of getting married. This is because that marriage is considered a type of rebirth for the couple, and because of the extra responsibility they accept for themselves. It's that link of the Jews accepting another festival on themselves as a type of renewal of vows to G-d that is so linked to marriage.
So, Tu B'Av is still philosophically an important day for Jews, even if not much is done to observe it. We hope though very soon the Temple will be rebuilt and we will again rejoice in the cessation of the cutting of the wood for the year.
Tu b’Av (èØåÌ áÀÌàÈá)
Question: What is Tu b'Av? Do we celebrate it today? What is the significance of the day?
The summer holiday of Tu B’Av (the 15th of Av) is a rabbinical holiday, one that the Mishnah describes in colorful detail:
Rabban Simeon b. Gamaliel said, “There were no happier days for the Israelites than the fifteenth of Ab and the Day of Atonement.”
What did they say?
During this dancing festival, men and women celebrated the grape harvest. It is remarkable such a day comes so soon after the 9th of Ab, the saddest day of the Jewish year. The 15th of Ab strongly suggests that the cure for exile lies in the power of love. The connection between the 9th of Ab and the 15th of Ab is as follows: think of the 9th of Ab like a pending divorce between God and Israel, whereas the 15th of Ab represents their spiritual reconciliation. Indeed the destruction of the Temple is analogous in many ways to the destruction of the nuclear home that occurs whenever two people divorce one another.
The 15th of Ab represents the willingness to start life anew, hence its imagery in rabbinical literature intimates forgiveness and renewal.
When looking at the Jewish folk traditions surrounding the 15th of Ab, it is always interesting to find parallels in other cultures—past and present.
For example: In Greek mythology, the father of Atalanta wanted his daughter to get married.
It so happened that Atalanta happened to be one of the greatest huntresses of her day; she was quick, and her mastery of the bow made her famous for she had won prizes in hunting and wrestling.
Atalanta really had no interest because she had served the virgin goddess Artemis. However, reluctantly, she agreed to honor her father's request—but on one condition: she would agree to marry only if one of her suitors could outrun her. She even gave them a head start, only to kill them when she passed them. Her father, King Schoeneus agreed. Unfortunately many of the possible suitors, they were killed until one young man, Hippomenes came along and asked the goddess Aphrodite for assistance. She gave him three apples and instructed him to leave an apple behind for Atalanta, and that is how Hippomenes won her hand in marriage.
From a modern perspective, the 15th of Ab bears a striking resemblance to Sadie Hawkins Day. In the Li'l Abner cartoon series, Sadie Hawkins was the daughter of one of Dogpatch's a fictional place) earliest settlers, Hekzebiah Hawkins, said to be the “homeliest gal in all them hills”, she grew frantic waiting for suitors to come a-courting her. After she became 35, her father became increasingly anxious. In desperation, he called together all the unmarried men of Dogpatch and declared it "Sadie Hawkins Day" (which happened to occur on Feb. 29th, during a leap year). In a special foot race, Sadie caught up to one of the town's eligible bachelors—and her days as a spinster were over.
The Talmud records other traditions and legends associated with this day. Historically, the tribes were allowed to intermarry with one another after the tribal boundaries been fixed and established (BT Bava Bathra 121a-b). Some say on this auspicious day, the Tribes of Israel finally allowed the Benjaminites to marry other tribes after nearly being exterminated by their brethren over the incident of Giveah. For many years, no tribe was allowed to marry anyone from the tribe of Benjamin (see Judges 21:18).
One rabbinical tradition records that on the 15th of Ab, the Romans allowed the Jews to marry the remains of the Jewish soldiers in the Betar fortress. Legend has it that the bodies of these fallen warriors did not decay.
In response to your original question, this much may be said: In general, many Jewish singles groups throughout the Jewish communities scattered around the world often have get-togethers on this day.
 Apollodorus, Bibliotheca 3. 9. 2 for Atalanta and 1.8.3 for the Boar Hunt.