In the Babylonian Talmud Tractate Ta'anit chapter 4, page 106, it is written:
"Rav Shimon Ben Gamliel said, 'There are no better days in Israel than Tu B'Av and Yom Kippur. On Tu B'Av, the daughters of Jerusalem go out dressed in borrowed white clothes, so those who lack garments are clothed, and they celebrate in the vineyards. And what do they say? 'Boy, raise your eyes and see what a selection you have! Let your eyes not gaze upon us ornaments, but let your eyes seek a family.'"
This idea of borrowing clothes from one another contributes to the respect for the poor and strengthens the feeling of unity and love amid the people. This is an unusual custom, and it is one that is not mentioned in reference to any other holiday. The Talmud goes on to describe additional aspects of young men coming to choose a soul mate.
This is the first description of Tu B'Av that we encounter, except for the description that appears in the Kitvei H'Ari.
On this day, we are promised that we will find soul mates and merit happy marital lives. But why on this of all days? The Talmud and the Ari explain connection between Tu B'Av and Yom Kippur. They say that a man who marries a woman for the sake of heaven -- whose marriage is not for the desire to receive for the self alone but rather to contribute to the tikun of the entire universe -- merits the same erasure of sins that takes place on Yom Kippur.
While some marry for beauty or wealth, there are also those who seek a soul mate for the sake of advancing the personal and global tikun process. A relationship for the sake of heaven supports Zeir Anpin consciousness, the desire to share abundance with the world, and therefore strengthens the entire universe.
Tu B'Av falls on the 15th of the month, when the moon is full and shining on us with the totality of the sun's Light without any interference. The fact that the moon appears full from the earth's perspective attests to the unification of Zeir Anpin and Malchut on the 15th day of the month. Therefore, the 15th of every month is ideal for new beginnings.
Tu B'Av is the best of all, because it is the day on which the full force of the sun, of Zeir Anpin, is revealed. The sun, being the source of power for the sign of Leo.
Leo is the most negative of the three negative months of the year. In these months we are cautioned not to begin anything new, as the Zohar tells us it is a dangerous period because of the high level of energy transmitted into the universe. The energy itself is not negative, but its tremendous strength is more than our vessels can handle without being damaged. This law applies to the entire month except for one day: Tu B'Av. On Tu B'Av there is complete unification between Zeir Anpin and Malchut, offering balanced cosmic support for every relationship and every new beginning. This is why it is a good day to marry and begin business ventures.
Most holidays require us to perform specific actions in order to connect to the Light, but on Tu B'Avthere is no need to observe any restrictions in order to manifest the potential concealed in the Light. The connection takes place regardless. On this day, we celebrate with just understanding Tu B'Av's significance. That's it; nothing else is required.
On this holiday, we can connect to the Light without any effort. Therefore, Tu B'Av is great for finding matches and great happiness.
Happy Tu B'Av!
Excerpt, "Days of Power"