The villages around Kasungu are part of the Chewa heartland, and Gule Wamkulu is performed at major ceremonies through the year. There is no fixed schedule for tourists; the dancers turn up at funerals, initiations, harvest festivals, and other significant occasions. If you are in the area at the right time, ask discreetly and respectfully — locals can sometimes arrange for visitors to attend.
Photography of the dancers in costume is generally not permitted, and the rituals around the masks are taken very seriously. Treat the performance with the same respect you would a religious service.