[@Cirrus] it's a type of lavish party that usually high class people do. You wear black or white or both. There's also strictly white party. Either way it started with Truman Capote. He decided in June 1966 to throw a lavish party. He was at the height of his popularity as an author and as a public figure following the publication of his non-fiction novel. For the first time Capote had the financial resources to host a party he deemed worthy of the friends he had cultivated in high society.

So basically nowadays celebs and rich people do it. Or people do it for the hell of it because it's a themed party.