Q. Can a Avel attend a Lag Baomer party by the fire where there is singing? Can he join in the dancing? Can he stay if instrumental music is played?

A. Nitey Gavriel (Pesach 3: 58: 15) rules that a mourner during the year of avelus is permitted to attend a Lag Baomer seuda (based on Remoh Y.D. 391:2) and also to partake on the singing and praising, but not on the dancing. He should also avoid attending if instrumental music is played. A similar opinion is found in Mekadesh Yisroel (Sefira 109). Other Poskim (Zichron Yitzchok p. 218, Lev Dovid 267) do not permit attending the seuda unless he was invited to say divrei Torah.
Horav Shlomo Miller’s Shlit”a opinion is similar to the Nitey Gavriel above, though he points out that the seuda should take part on Lag Baomer itself and not in the night after.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a