At the beginning of the month of Av, the mourning over the destruction of the Temple intensifies, and the Sages said that one should reduce joy from Rosh Hodesh onward. Nevertheless, joy occupies a central place in hasidic thought, and it is explained that it is pain and worry that need to be decreased, by rejoicing.

“When Av begins, one reduces joy” (Mishna Ta’anit 4:6); the [hasidic] interpretation is: We reduce sadness and worry with joy, meaning: By rejoicing. Through the joy that a person has, he lessens the exile of the Shekhina. (Rabbi Kalonymus Kalman Epstein, Ma’or VaShemesh, Vayeshev)