Rabban Simeon ben Gamaliel said: Never were there any more joyous festivals in Israel than the 15th of Av and the Day of Atonement, for on them the maidens of Jerusalem used to go out dressed in white garments--borrowed ones, however, in order not to cause shame to those who had none of their own. These clothes were also to be previously immersed, and thus the maidens went out and danced in the vineyards, saying: Young men, look and observe well whom you are about to choose...To archaeologists, it is a semi-Dionysian midsummer festival, celebrating fertility and fermentation. To romantics, it is the Jewish Valentine's Day. To traditionalists, it marks the repeal of various harsh decrees, as recorded in the Talmud ad loc. (30b-31a)--at most, a partial consolation for the ancillary tragedies of 9 Av.
However, the final answer in the Talmud (the only one voiced by two sages) may give us a clue. This relates it to the cutting of wood for the Altar. Earlier in the chapter, the Mishna lists a number of days set aside for different distinguished groups to bring wood, the simplest of donations, to the Temple. However, the 15th of Av is special because this is when "the priests, Levites and anyone who was mistaken about his tribe, and the family of the pestle-smugglers and the family of the fig-pressers" would bring. The priests and Levites mentioned here do not seem to be the high-ranking and wealthy Sadducees, as they would not associate with the unwashed masses. Tu beAv is the day for even the lowliest minimum-wage earner to bring something to the Temple. In fact, Megillat Taanit, which predates the Mishna by a century, lists this day specifically as a happy one. Its scholium (commentary) refers to this as the day designated for "the family of priests and Levites, converts, serfs, bastards and freed slaves" and notes that originally the day set aside was 9 Av, but with the great numbers of exiles returning, the Sages pushed it off a week, until the 15th. In Greek, it is called Xylophory, the Day of Wood-bearing.
However, it is at the time of the Great Revolt, in the year 66, when this day becomes truly remarkable. Josephus Flavius records (Wars of the Jews, Book II, Chapter 17):
5. Upon this the men of power, with the high priests, as also all the part of the multitude that were desirous of peace, took courage, and seized upon the upper city [Mount Sion;] for the seditious part had the lower city and the temple in their power; so they made use of stones and slings perpetually against one another, and threw darts continually on both sides; and sometimes it happened that they made incursions by troops, and fought it out hand to hand, while the seditious were superior in boldness, but the king's soldiers in skill. These last strove chiefly to gain the temple, and to drive those out of it who profaned it; as did the seditious, with Eleazar, besides what they had already, labor to gain the upper city. Thus were there perpetual slaughters on both sides for seven days' time; but neither side would yield up the parts they had seized on.This helps us understand why Tu beAv is such a mystery in the Mishna. It represents the height of Jewish victory against Rome; under Caesar's rule, it can hardly be celebrated as such. The mourning of Tisha beAv conveys the foolhardiness of rebellion, but rejoicing on Tu beAv? That had to be concealed. Nevertheless, the core of it, the erasure of social barriers and the celebration of Jewish survival, remains to this day.
6. Now the next day was the festival of Xylophory; upon which the custom was for every one to bring wood for the altar (that there might never be a want of fuel for that fire which was unquenchable and always burning). Upon that day... they grew bolder, and carried their undertaking further; insomuch that the king's soldiers were overpowered by their multitude and boldness; and so they gave way, and were driven out of the upper city by force. The others then set fire to the house of Ananias the high priest, and to the palaces of Agrippa and Bernice; after which they carried the fire to the place where the archives were reposited, and made haste to burn the contracts belonging to their creditors, and thereby to dissolve their obligations for paying their debts; and this was done in order to gain the multitude of those who had been debtors, and that they might persuade the poorer sort to join in their insurrection with safety against the more wealthy; so the keepers of the records fled away, and the rest set fire to them. And when they had thus burnt down the nerves of the city, they fell upon their enemies; at which time some of the men of power, and of the high priests, went into the vaults under ground, and concealed themselves, while others fled with the king's soldiers to the upper palace, and shut the gates immediately; among whom were Ananias the high priest, and the ambassadors that had been sent to Agrippa. And now the seditious were contented with the victory they had gotten, and the buildings they had burnt down, and proceeded no further.
Isn't it time we start realizing what Tu beAv is really about?