The first time the 15th of Av is mentioned in the Mishnah is in the tractate of Ta'anit, mentioned as one of the two most exceptional holidays of the year, alongside Yom Kippur. On these two dates the girls of Jerusalem would go out and dance in the vineyards dressed in white clothes.  The Babylonian Talmud lists six positive events that took place in Jewish history on the 15th of Av:

 

1. The day the Jews ceased to die in the desert: Every year of wandering in the desert, on the eve of the 9th of Av, the Jews would dig graves and lie down inside them. In the morning, some of them were found dead in their graves. On the last year the Jews were in the desert, since no one was found dead in his grave, they continued to lie in the graves the following nights considering they may have erred about the date; this they had done repeatedly until the night of the full moon, the 15th of Av, when it was concluded that the date had actually passed and the decree was over.

2. The day the tribes were allowed to intermarry: Following the settlement of the tribes in the Land of Israel, the custom of refraining to intermarry between tribes out of fear that inherited land would transfer from tribe to tribe, had come to an end.

3. The day the Tribe of Benjamin was allowed to intermarry with the rest of the tribes: As part of the civil war against the Tribe of Benjamin following the incident of the Concubine in Givah, the tribes of Israel swore they would not marry off their daughters to the sons of the tribe of Benjamin. In order to prevent the tribe of Benjamin from perishing following the annihilation of all the women of the tribe who were killed in the civil war, the other tribes had permitted the surviving men of Benjamin to marry women from among those who celebrated the ancient dancing festival that took place in Shiloh. Possibly the similarity of this holiday to the holiday as described in the tractate of Ta'anit is the source of this commentary.

4. The day wood offerings were no longer brought to the Temple: This was the date wood was no longer chopped for the purpose of placing it on the altar for burning offerings in the Holy Temple. This - due to changes in weather conditions at this time of the year turning the trees damp to the extent that worms would appear on them rendering the cut wood invalid for using on the altar.

5. Removal of the patrols stationed by Jeroboam son of Nabat: According to this tradition, when the kingdom was split in the days of Rehoboam (the son of Solomon), Jeroboam the son of Nabat stationed patrols that prevented the tribes of Israel from ascending to the Holy Temple in the Jerusalem, and these were removed by Hoshea the son of Elah, the last king to rule Israel prior to the destruction of the kingdom of Israel and the exile of the ten tribes. If we accept this tradition, the altars built by Jeroboam the son of Nabat in Bet – El and in Dan, were the major centers of worship for the nation of Israel, excluding the years of King Solomon's reign and the latter years of the Kingdom of Israel.

The day permission was given to bury the Jewish dead of Beitar: According to the tradition, Emperor Hadrian decreed that the Jews killed in Beitar during the Bar Kokhba revolt were not to be buried, and only when he was replaced by a different emperor, did the latter allow the burial of the Jews. Despite the time that had elapsed since their demise their ​​bodies did not give off a foul scent.