On Tu B'Av, the fifteenth of Av, we observe a partial holiday. We don't say "Tachanun," a daily prayer for Hashem's mercy. In addition, bride and groom do not fast on this day, as is customary on the day of one's marriage. These customs commemorate the many happy events that occurred on this date in Jewish history.
The last Mishnah in Masechet Taanit says, "There were no holidays so joyous for the Jewish People as the Fifteenth of Av and Yom Kippur, for on those days, daughters of Yerushalayim...(more