costume party
costume ball
fancy dress party BrE
fancy-dress party BrE
fancy-dress ball BrE

A third for a costume ball at the home of an earl.

He quickly discovered that it was not exactly a costume ball.

Had she ever attended his mother's annual costume ball before?

"Are you gentlemen on your way to a costume ball?"

You look like you've been at a costume ball.

The moonlight was magical the night of the costume ball.

I was especially impressed with the piece about the costume ball.

Towards the end of the Atlantic crossing, the traditional costume ball took place.

It looked to Kyria as if they were dressed for a costume ball.

I suppose you can say it's a costume ball.