Our son z’l passed away six months ago. Our niece is getting married in a month. Are we allowed to go to the wedding and stay during the reception where a band will be performing?

Avelut on parent's lasts a whole year, but for other close relatives we only have one month of Avelut. If the occasion is not within the Shloshim (the first month of the Avelut), there's no problem at all with you participating.