The Gemara in Bava Batra, 121a, wonders why the 15 day of Av is a day of celebration, since it's not written in the Bible. A list of answers are given, one being that at the end of the war between the tribe of Benjamin and all the others (Judges 25) a vow was made never to intermarry with the sons of Benjamin; a later generation decreed, on the 15th of Av, that the vow had been for that generation, not all generations, and it was time to bury the hatchet.
Rashbam, a 12th century scholar and grandson of Rashi, fills Rashi's usual role of primary interpreter of the Gemara for this tractate. He brings the following story, from Midrash Eicha, itself a Talmudic-era exegesis:
The day the Israelites accepted the warning of ten of twelve scouts newly back for the Promised Land, whereby the Canaanites were too strong to be overcome, was the 9th of Av. Faced with this rebellion, God decreed that the 9th of Av would henceforth be a day of mourning; He also decreed that the people would wander an extra 38 years in the desert, and that none of the adults of the time would live to enter the Promised Land except Joshua and Kaleb, the two loyal scouts of the twelve.
Every year thereafter, on the night of the 9th of Av, all the adults would dig graves and sleep in them. The next morning a voice would call for them to rise, but a portion of them wouldn't - a 40th (actually, it should have been a 38th) would have died. On the 40th year, as preparations were being made to enter the Promised Land, the adults dug their graves and laid down in them; in the morning they were all still alive. Fearing they had miscalculated the date, they lay in their graves the next night, and the next, and the nex; each morning everyone was still alive. On the 15th there was a full moon (the Jewish calender is lunar), so clearly they hadn't miscalculated, and the 9th had passed without mass death. The curse had ended,and the 15th became a day of rejoicing.
This thread began, and is explained, here. Though it occurs to me to add another bit of explanation. In a way, one can say that the Jews as a group are the sum of their communal memories. Faced by thoughtless, ignorant but malicious types such as Caryl Churchill and the many millions of her fellow angry fools, these occasional Daf Yomi posts are meant to give glimpses into the richness and complexity of Jewish culture and memory.