Laws of the Three Weeks and Chodesh Av (5)
Tu B’av: Fifteenth Day of Av. The Mishna (1) states in the name of Rabban Shimon Ben Gamliel that during the Bais HaMikdash there were no days as festive as Yom Kippur and the 15th day of Av. A number of salvations happened on this unique day (2).
By interpreting the posuk correctly, Bnei Yisroel realized that the issur (3) of a woman inheritor, marrying out of her tribe, was only for the first generation. This made shidduchim easier and was a reason for it to be a Yom Tov.
Shevet Binyamin, after being somewhat expelled from Klal Yisroel due to a sin, was allowed back into the fold. Thus, other tribes could again marry into Shevet Binyamin.
Those destined to die in the desert stopped dying.
Hoshea ben Elah removed the roadblocks that Yeravam ben Nevat made to stop Yidden from going to the Bais HaMikdash each Regel.
After the Beitar massacre, the Romans did not let Jewish corpses be buried; miraculously the bodies did not decompose. On this day, permission was given to bury all the bodies. On this day, the Sages in Yavna added a fourth bracha to bentching after bread meals - Hatov [the Good One] Vehameitiv [the One who does good to others]. Hatov refers to the chessed that the bodies did not rot and Hameitiv to the chessed that they were buried. NOTE: When one bentches, he should have in mind to thank Hashem for these two chasadim. To understand the importance of thanking for these chasadim often and forever, see Meshech Chochma (4).
The mitzvah of cutting down trees for the Mizbeach ended as the days began getting shorter and the sun could not dry out the trees enough to be suitable for the Mizbeach. NOTE: from here we learn the concept of celebrating the completion of a mitzvah that takes a long time to complete (5).
New Season. The Gemara (6) says that now with the longer nights, one who adds time to his Torah learning at night will add to the length of his life (7). This is also the turning point [last 45 days of year, 40 days before Elul 25, day the world was created] of the year when we start to think about finishing off the year in a mode of aliyah. Some have a minhag to start wishing from this day on a Ksiva Vachasima Tova, while others wait till Rosh Chodesh Elul
(1) תענית כו: (2) שם (3) במדבר לו-ח,ט (4) משך חכמה דברים ח:י (5) עיין נמוקי יוסף ב"ב קכא. (6) תענית כו: (7) הובא במשנה ברורה רלח:ב