I've heard of this event before. But, it was where house slave (mixed people) went to meet others like themselves.
Other house slaves (who were by no means all mixed) went to meet other house slaves, or other members of the mulatto community?
Do you remember where you heard this?
The mulatto community around NO in the antebellum period was quite varied and several families achieved rather notable social standings, even in the larger community. IIRC, the Mettoyer (Nettoyer?) family had quite the large fortune, with Mrs. Mettoyer (a mulatto) being one of the largest slave owners in the south at the time of the War between the States.
It's fascinating, really.