Wiktionary
costume parties
n. (costume party English)
Usage examples of "costume parties".
She couldn't let it be sold to some sophisticated young modern, who would giggle over its quaintness and wear it to costume parties.
I was wearing my best suit, but my best suit was the kind the inhabitants of Xanadu would wear to costume parties or give away to the Salvation Army.
A woman chained to one home and family may redecorate frantically or take a lover or go to many costume parties.