Modern day school dance
A very large orgy, in which all participants wear formal clothing. Although, I can tell you from experience that wearing no clothes at all helps with the purpose of the orgy.
Joe: So, Fred, did you go to the "modern day school dance" Saturday?
(Fred gives a puzzled look)
Joe: The giant orgy?
Fred: Ooooh, yeah!
(Fred gives a puzzled look)
Joe: The giant orgy?
Fred: Ooooh, yeah!