Arising from the fast and mourning of Tisha B'Av (the Ninth of Av) comes a time of joy, love and consolation. The Shabbat following the Ninth of Av is known as Shabbat Nachamu (the Sabbath of Consolation) based on the first verse of the Haftarah reading, Isaiah 40:1, "Console, console my people, says your God."
This Haftarah begins what is known as "the Seven of Consolation," the seven weeks between Tisha B'Av and Rosh Hashanah. Each of these weekly Haftarah readings deal with prophecies of consolation (as opposed to the three Haftarot prior to Tisha B'Av which are known as "the Three of Affliction). These Haftarot, all taken from the Book of Isaiah, spread from the Torah readings of Va'etchanan to Nitzavim. Since Lamentations is read on the Ninth of Av uses the same Hebrew root for "weep" in two forms, symbolizing weeping for both the First and Second Temples, so too, we are offered double consolation after the fast since Isaiah 40:1 mentions "console" twice.
Another more less-known celebration after Tisha B'Av is Tu B'Av (the Fifteenth of Av). According to the Talmud in b.Taanit 26b, several events occured on this day.
First, 40 years after the generation of the Exodus, the last of that generation passed and the death of that generation was ceased. A second event that occured on this date in history was that the tribes of Israel were once again allowed to inter-marry with members of other tribes within Israel. The ban was instituted so as to ensure a proper division of the Land of Israel. Still another event that happened on this day was the ban on marrying into the Tribe of Benjamin (see Judges ch. 20-21), was lifted. Also on the 15th of Av, Hosea ben Elah removed all roadblocks that were established by Jeroboam ben Nevat; these roadblocks, which prevented people from the northern kingdom from making the required pilgramages to Jerusalem, were established after the death of Solomon when the land of Israel was divided into two kingdoms. Hosea destroyed these roadblocks 200 years later. A fifth occurence on Tu B'Av occured when the Temple stood. It was on this day that the firewood was cut for the Altar for the upcoming year of Temple service. Also on this day, after the Bar Kochba rebellion, the people who died at the fortress of Betar were allowed to be buried.
The Talmud also records another event that occured on Tu B'Av. It was on this day that matchmaking occured. It is said that on this day the daughters of Jerusalem would go out and dance in the vineyards during which they would tell the young men to make a choice for themselves concerning a bride to take.
The observances for Tu B'Av are limited since the Temple is currently not standing. The Tachanun (pentential) prayers are not said from Minchah the previous day and all throughout the 15th of Av. Also, as with other festival dates, both brides and grooms who are to be married on this day do not fast. On the 15th of Av one is encouraged to increase his Torah study since the days are longer during the summer months. Also, being a prelude to the month of Elul, it is wise to begin taking introspection of one's life in preparation for the upcoming High Holidays.
In modern-day Israel, Tu B'Av is also celebrated as a holiday of love (Hebrew: חג האהבה, Chag HaAhava), similar to Valentine's Day. It has been said to be a "great day for weddings".