"It can be either. Sometimes people simply feel like having a night of dressing up, although masquerades are usually less common that normal parties even then."

Possibly because of the fact that there's more effort that goes into them, if only on account of the fact that people need to find or otherwise come up with costumes. Not to mention the effort that can be involved in bringing one's costume to the particular venue that's been chosen for the party.

"In this case, however, I would imagine that it's because of the closeness to, well, consider it a sort of harvest festival. One that's evolved a good deal from its initial roots, yes. But it is about the season for that kind of thing all the same."