Brief chronology of the film:
Brad and Janet attend Hapschatt wedding
Brad and Janet try to visit Dr. Scott; get stuck at Frank's castle instead
They meet Frank and co, see Rocky's "birth", and watch Frank murder Eddie
Brad and Janet are shown to separate bedrooms for the night
Rocky escapes, Janet finds him and they fool around, Dr. Scott shows up, Brad, Frank, and others discover Janet and Rocky in flagrante delicto.
Frank decides to serve dinner
Upon being informed that Eddie was the main course, Janet runs into Rocky's arms; Frank gets jealous and chases her away; Scott and Brad chase Frank.
Frank freezes everyone except himself, Riff-Raff, and Magenta, dresses them up, and starts the floor show
Riff-Raff and Magenta appear, and assume command from Frank; Riff-Raff kills Columbia, Frank, and Rocky.
Dr. Scott, Brad, and Janet flee the castle; it returns to Transylvania; the movie ends
The important part, for our purposes, is that dinner is served after everyone had gone to bed. Granted, everyone got up and left their rooms, but no one knew that would happen in advance. So Frank seems to have scheduled a dinner for five people (six, if he somehow anticipated the sudden arrival of Dr. Scott), and arranged for it to be served while four of the five guests were in bed. Furthermore, it was a birthday dinner, and the birthday boy wouldn't have been present.
