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1. There are various opinions about why the fifteenth of Av is considered one of the two happiest days of the Jewish year.
2. There was great rejoicing when the casualties of Beitar were allowed to be buried.
3. When the nights start getting shorter again, people must actively start learning more Torah at night.
4. The custom was that all unmarried girls would borrow dresses for the Tu b'Av event.
5. The Gemara describes the Tu b'Av event.
A BIT MORE
1. The other happy day of the year is Yom Kippur, when we are forgiven for our sins.
2. The Romans did not allow their victims, the Jews of Beitar, to be buried for a long time. When they finally granted permission for burial, the Jews who came to bury them were comforted by the fact that their bodies had miraculously stayed fully intact and had not decomposed.
3. This happens every year around the fifteenth of Av. One who starts learning more at night on these days will be blessed with a long life, while one who does not will die before his time.
4. This was in order not to embarrass a girl who did not own a dress.
5. Each girl would point out her attractive qualities, whether they were her looks or family lineage, and she would also mention the fact that someone should just marry her for the sake of Heaven. (If a boy of marriageable age liked one of the girls he saw, they would see if they liked each other, talk it over with their relatives, agree to terms and get engaged. -Me'iri.)
Index to Revach for Maseches Ta'anis