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. He studied human ethnography and used a holodeck to create an appropriate venue and characters. After the luau, one of the holograms, Marayna, transferred herself to The Doctor's mobile emitter and appeared in Tuvok's quarters. When Tom Paris went to investigate the holodeck malfunction, he found himself back in the luau program now being controlled by Marayna, and commented it was "kind of creepy." (VOY: "Alter Ego")