What are the parties where very rich people all dressed up, listen to classical music and dance Waltz called?

Those were called "balls." https://en.wikipedia.org/wiki/Ball_(dance_party)

Gatsbys?🤔

Boring parties

Lol, my aunt would have parties kinda like that. Like, charity banquet thingies. I guess they're kind of like a ball, like the one in Cinderella.

ball parties?

Keggers