The Mishneh states: Rebbe Shimon Ben Gamliel said, “There were not days of joy for the Jewish people like the 15th of Av and Yom Kippur.” These days became a day of Shidduchim, where men would pick for themselves brides. The potential brides would all go out with white clothing and tell the bachelors to choose for themselves a girl of good character and lineage and not look at matters of vanity, such as beauty. The reason for the great celebration on Yom Kippur is because a) It is a day where we are forgiven for our sins and b) Moshe received the second Luchos.
 Taanis 26b
 Gemara Taanis