Rabbi Moshe Weinberger, Rabbi of Cong. Aish Kodesh in Woodmere, NY, and Mashpia at Yeshiva University, tells the following story every year to his congregation when Tisha be'Av comes around (heard from Rabbi Weinberger). You can hear it here, around 16-17 minutes into the recording.
The Koidenover Rebbe (he uses the term Koidenover tzadik, but I think it means the same thing here) used to dance on Tisha Be'Av. His puzzled Chasidim asked him about it. He responded to them as follows. Tisha be'Av there is a mitzvah to be be'aveilus (in mourning for the Beis Hamikdash). And we know that every mitzvah must be done besimcha (with joy). That is why I am dancing.
Now that is an interesting story, but the whole thing does not add up, due to the following
a) Do Rabbi Weinberger himself, and his congregation, dance on Tisha be'Av, and follow the way of the Koidenover Rebbe? If not, why not? If he holds it is correct, why not do so? And if he holds it is not correct, why does he repeat it every year?
b) According to this story, people should dance at a levaya (funeral), and burial as well, as well as when visiting a shiva house for nichum aveilim (condolence call). After all, those are mitzvos too. Do they do so?
Rabbi Weinberger states, regarding this story, that 'the Misnagdim bichlal can't hear it'. Well, maybe the Misnagdim can't hear it, because it doesn't add up, as above. So it is actually a praise for the Misnagdim, that they don't accept it!
It seems like this is another cute Chasidic story that should be discarded, as it doesn't add up. Even among Chasidim, I am not aware of anyone who actually follows the story and actually dances on Tisha Be'Av.
File it away in the recycle bin.