# It was the day that the wood that was donated during the summer for the Mizbe'ach in the Bet Hamikdash would be completed.
# It was the day on which the Jews were given permission to bury their dead in Beitar after the Romans massacred them and didn't let them bury their dead.<ref>Gemara Tanit 30b</ref>
According to the Zohar<ref>Zohar vol. 2 page 195</ref>, one must make Simcha on Tu BeAv because of Ilui Shechinah.<ref>Moreh BaEtzbah 8:242, Ben Ish Chai (Shanah Sheniah, Devarim 1)</ref>
According to the Bnei Yisaschar<ref>Mamarei Tamuz - Av n. 3 s.v. bpasuk vav, s.v. vtavin, Mamarei Tamuz - Av n. 2 s.v. vheneh ket</ref> all the reasons that Chazal give for Tu Be'av revolve around reconciliation and harmony, whether it is between the Jews one to another or between the Jews and Hashem. It is the correction of the sins of [[Tisha B'av]]. Also, it is the 28th day from [[Shiva Asar BTamuz]] which is the gematria of כח, strength, as we hope that the incomplete names of Hashem which add up to 22<ref>