R. Yosef says in the name of R. Nachman that this was the day that the tribe of Benjamin was [again] permitted to marry the daughters of the other tribes. [This Gemara is based on the passage in Judges (ch. 19-20) regarding pilegesh beGiv’ah – the concubine at Gibeah, a woman who, together with her common-law husband, went to Gibeah, a town within the land of the tribe of Benjamin. Gibeah was populated by ruffians who wished to molest the husband but wound up abusing the woman. She died as a result of their brutality. The subsequent outcry throughout Israel instigated a war that caused the tribe of Benjamin to reach near devastation. The result was that the men of the tribe who survived had few matches to contemplate with the even smaller number of women that survived but, as the verse states (Judges 21:1), “Ve’ish Yisrael nishba bamitzpa lemor, Ish mimmenu lo yiten bito le[B]inyamin le’isha ? The men of Israel had taken an oath at Mitzpah saying, ‘None from among us will give his daughter as a wife to [the tribe of] Benjamin.'”
The Gemara asks, “What was the Scriptural source that permitted them to lift the prohibition?” Rav says that the verse states “mimmenu ? from among us,” but not “baneinu – [from among] our children.”
Rabbah b. Bar Chana said in the name of R. Yochanan that the 15th of Av was the day on which the generation of the wilderness (those who had come out of Egypt and were wandering in the desert for 40 years) ceased to die out. For a master (a sage) said that as long as the generation of the wilderness continued to die out there was no Divine communication with Moses. Rashi (s.v. “Lo haya hadibbur im Moshe”) explains that there was no direct and endearing conversation. All the other conversations in that time period were not “peh el peh ? lit. mouth to mouth”, but rather visions at night, as the verse explains (Deuteronomy 2:16-17), “Va’yehi ka’asher tamu kol anshei hamilchama lamut [mikerev ha-am], Va’yedabber Hashem elai lemor … – When it came to pass that all the men of war finished dying [from amidst the people], Hashem spoke to me, saying…” The Gemara explains this to mean that only then did the Divine communication resume directly “to me,” i.e., Moses, and this happened on the 15th of Av.
Ulla gives another reason for the uniqueness of this day, stating that this was the day that Hoshea b. Elah, who reigned in Israel during the time that Ahaz was king of Judah (II Kings 17:1), removed the guards which Jeroboam son of Nebat, the first king of Israel, who had split away from the kingdom of Judah, had placed on the roads to prevent the people of the kingdom of Israel from ascending to Jerusalem for the festival pilgrimages. The Gemara (Gittin 88) explains that Hoshea b. Elah was also a wicked king, but not as wicked as other kings of Israel, for while he permitted pilgrimages to Jerusalem, he still allowed the people to worship idols.
R. Mattenah offers another reason as stated in Tractate Gittin (57a, Perek Hanizakin). It was the day when permission was granted to bury those killed at Beitar during the Bar Kochba revolt. The Gemara relates how the town’s inhabitants, including men, women, and children, were slain and their blood flowed for seven years. The 15th of Av was the day that permission was granted to bury those killed at Beitar. In commemoration, the Sages in Yavneh instituted the blessing of “Hatov VeHameitiv – Hashem, who is kind and deals kindly,” because the dead corpses had not putrified and because permission was granted to bury them. This blessing was permanently added to the text of Birkat Hamazon – the Grace after Meals. Yet another explanation is found in the Gemara: Rabbah and R. Yosef both say that this was the day on which [every year], they stopped felling trees for firewood for the altar. This is deduced from a baraita where R. Eliezer the Elder says that from the 15th of Av and on, the rays of the sun weaken and trees cut for firewood would not dry sufficiently. Rashi (s.v. “Milichrot”) explains that since the wood was still moist, it might harbor worms, making it unfit for the altar, as stated in the mishna (Middot 2:5). R. Menashya said that they called the 15th day of Av “the day of the breaking of the axe.”
The Gemara also relates that on the 15th of Av the daughters of Jerusalem would go and dance in the vineyards in borrowed white garments so as to be dressed alike and appear to be of the same social status. Young unmarried men would come there to find a match.
The 15th day of Av has some practical halachic implications as well. We do not fast, make eulogies, or say Tachanun. If the 15th of Av falls on Shabbat, we do not recite Hazkarat Neshamot, nor do we say Tzidkat’cha at Mincha. Tu Be’Av is a joyous time in its own right. May we speedily come to the time when the days of fasting and mourning will become holidays as well.