Who: Anyone and everyone... so long as you're legal.
Not a celebrant of Saturnalia? Who cares. It's a reason to party! And who doesn't like a party this time of year? There will be food. there will be drinking. There will be entertainment.
But there's a catch!
Saturnalia is marked with Role-Reversal... And so everyone must show in costume. Not just any costume - a costume depicting something that is your opposite. Masters may come as servants! Men can come as women! If you're particularly extraordinary, come dressed as a normal Joe! Whatever you like, so long as it's something you're not.
Come, have fun, and enjoy yourselves in this lovely world we've been given.