What is Tu B'av all about? While many Divrei Torah focus exclusively on the unusual courtship ritual described in the Mishnah and the six (!) explanations offered for the holiday in the Gemara, a deeper understanding emerges from viewing Tu B'av in its wider context in Masekhet Taanit.
The opening Mishnah of the 4th chapter discusses three fasts that appear similar at first glance: Tisha B'av, the ma'amadot (fasts of Israelites who represent the community at the Temple) and Yom Kippur. Indeed, they share a common denominator: on only these three occasions it was customary to recite Birchat Kohanim at all four daytime prayers, including Neilah.
The chapter goes on to demonstrate, however, that Tisha B'av and Yom Kippur are in fact opposites. Tisha B'av is a day of mourning, Yom Kippur of joy. Appearances are deceiving. Two people can be dressed up in black suits, white dress shirts and black ties; one attends a funeral and the other a wedding.
Tu B'av represents the same theme. The Jewish girls go out in borrowed clothing, so as not to embarrass one another. We can no longer distinguish the rich from the poor, the beautiful from the poor. Their garments are all immersed in the Mikvah; they too are all now equally pure. The ladies call out to the men who have gathered: don't look at beauty; beauty is deceitful. Look instead at the family and the God-fearing character the young lady represents.
The tractate concludes with the same message. The verse from Shir Hashirim refers to "the day of his engagement and the day of his joyous heart." The verse, explains the Mishnah, is not to be taken literally. The betrothal is the Revelation at Sinai; the day of joy is when the Temple was built. Not everything is as it seems. The words of the verse - like the young Jewish men and women - carry much deeper meaning than any cursory once-over could ever reveal.
The tragedies detailed in the Mishnah reflect the opposite. The sin of the Golden Calf, for which the Jews were forgiven on Yom Kippur, was due to the Jews being unable to look beyond the concrete. They failed to imagine a God that did not require physical manifestation, and so they built the Calf. Idolatry, which was rampant during the waning years of the First Temple period, was born of a similar inability to move beyond the need for icons. On the original Tisha B'av, the Jews took the spies' report at face value. They gave up hope instead of looking beyond the surface and digging deeper. It was due to sinat chinam (baseless hatred), a shallow inability to look beyond our friends' actions and empathize with their true intentions, that the Temple was destroyed.
The tragedy of Tisha B'av was born in our inability to look beyond the surface. The pure joy of Tu B'av comes from our ability to correct the sins that led to the destruction. The mitzvah of this day is to penetrate beneath the surface and see the profound depth in the Torah that we study and the people with whom we interact each and every day.