Q. We have the minhag that we usually dance and sing after Kiddush Levana. Is it correct to do so on Motzei Tisha Beav also, since the avelus (mourning) still applies until the afternoon of the next day?
A. Remah (O.H. 426: 2) mentions the dancing done after Kiddush Levanah based on Meiri (Sanhedrin 42a).
Since the dance and singing is part of a mitzva it is permitted, as eating meat would be. (Nechmas Yisroel (2: 3) quoting Horav Eliashiv zt’l, Nitei Gavriel- Bein Hametzarim 1: 49: 4 – 2: 87: 9 – Yerushalaim Bemoadeiha p. 232).
Other Poskim disagree and recommend not to sing and dance (Halichos Shlomo 15: 22, Teshuvos Vehanhogos 1: 206 quoted by Yeshuos Daniel p. 19).
Horav Shlomo Miller’s Shlit’a opinion is that each one should maintain his own traditions.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a