The first mention of Tu b’Av (15th of the Hebrew month of Av) as a special day in Jewish sources is in the Mishnah (end of the 2nd century): Said Rabbi Shimon ben Gamliel: There were no greater celebrations for Israel than the 15th of Av and Yom Kippur. (Talmud, Taanit 26b).
According to the Talmud, on the 15th of Av the daughters of ancient Israel would go out dressed in white and dance in the vineyards, saying young man, consider whom you choose (to be your wife)?”(Ta’anit, Chapter 4). In other words, in early generations this day was a day of marriage or that which would ultimately lead to marriage, and since, according to Jewish tradition, a groom is forgiven for his sins on his wedding day (similar to Yom Kippur), both of these days are a type of celebration.
In addition, there were several more joyous events in Jewish history which this day commemorates, including:
1) Several months after the people of Israel were freed from Egyptian slavery, the sin of the spies lead to the decree that the entire generation would die in the desert, and that their children would enter the land instead (Numbers 13 and 14). After 40 years of wandering through the wilderness the decree was annulled and the next generation stood ready to enter the Holy Land. It was the 15th of Av of the year 1274 BCE.
2) On this day in history, the tribes of Israel were permitted to intermarry. In order to ensure the proper division of the Land of Israel between the twelve tribes, the generation that conquered and settled the Holy Land was restricted to marrying within their respective tribe, and preventing the transfer of lands between tribes. This restriction was lifted on the 15th of Av, which was considered a cause for celebration and festivity.
3) The fortress of Betar was the last holdout of the Bar Kochba rebellion. When Betar was captured on Av 9, 133 CE, thousands of Jews were killed and the Romans and they were forbidden to bury their dead. After they miraculously did not decompose, the dead of Betar were finally brought to burial on Av 15, 148 CE.
In addition, according to Jewish tradition, the 15th of Av marks a preface to Elul, the month of preparation for judgment on Rosh Hashanah, and it is therefore proper for a person to begin to review his actions during the year.