On Earth, a luau was a Polynesian-style feast whose participants traditionally wore Hawaiian shirts, leis, and other tropical dress.
Neelix hosted a luau on USS Voyager in 2373. Tom Paris criticized Harry Kim's initial choice of attire, saying he looked like he was attending a funeral instead of a luau. Kim explained that he was doing a Vulcan meditation. Later, he arrived properly clothed. (VOY: "Alter Ego")