R. Shmuel Bar Nachmani said in the name of R. Shmuel Bar Yitzhak: The day on which the tribes were allowed to intermarry (Numbers 36) and every daughter who inherits land etc., and it is written (ibid., 36):  No inheritance in Israel is to pass from one tribe to another etc.; is it possible for a daughter to inherit from two tribes? Rather, from now say that her father is from one tribe and her mother from another, and the Rabbis said, the day the Tribe of Benjamin were allowed to marry into Israel, as it is written (Judges 21): Cursed be anyone who gives a wife to a Benjamite... 

​

Rav Yehuda said that Shmuel said: the day on which the members of different tribes were permitted, Rav Mattana said: the day that the...   slain of Beitar were brought to burial, Rabbi Eliezer the Great says: From the fifteenth of Av onward, the strength of the sun grows weaker, and from this date they would not cut additional wood for the arrangement, and Yasif said, R. Ab in and R. Yochanan taught: The 15th of Av is the day upon which the grave digging of the dead stopped. R. Levi taught: On the eve of the 9th of Av, Moses would announce throughout the camp: Go dig! The people of Israel would go and dig their graves and sleep inside them. When morning came, Moses announced: Let those who live separate themselves from the dead. The live ones would rise, separate themselves from the dead, and count –and each year 15,000 would die. On the 40th year since leaving Egypt they did the same thing. But in the morning –none had died. They said: Perhaps we had miscalculated the date? They thought, and slept in their graves again the following night, on the 10th of Av, and so again on the 11th, and on the 12th, and on the 13th, and on the14th –When the moon became full on the 15th, they declared: The Holy One, Blessed be He, canceled the decree. They made this day into a holiday.