There is no such thing as "Tu B'av". You have combined two religious observances into one name. We celebrate Tu B'shvat as the "New Year of the Trees", the re-awakening of the foliage that had died during the winter months. It is joyous and not at all a religious observance.

Tisha B'Av, on the other hand, is a day of commemoration for, yet, another attempt at destroying the Jewish people and it is one on which we sit low to the ground as a sign of mourning. We read from the book of Lamentations and reflect on the seriousness of these types of past events and the miraculous fact that we are still an intact people despite them.