Seeing as Magenta and Riff are aliens, they have no idea about earth culture, so they probably just wanted to eat in the middle of the night for the hell of it. Look at the expression on Frank's face after Magenta says, "Master, dinner is prepared!" They don't know that dinner isn't supposed to be eaten then.
(Also, did anyone notice that at the dinner scene, somebody has a jar to drink from instead of a wineglass? I thought that was so funny!)
i love the time frame this movie is in. it begins late at night at the castle. they have their little hoopla with rocky, and retire to bed. theyre in bed for some time (janet had sex with frank and so did brad, that takes SOME time). then janet has sex with rocky. its still dark. by now its probably early morning. then they eat dinner! they have the floor show, that takes some time....then they end up outside after the castle lifts up...its STILL dark outside.