Billy will be arriving reasonably on time and probably with Teddy (and/or Tommy) - or not far apart from each others' arrival, at the very least - bearing an enormous bowl of salad because even magic can't save him in the kitchen most days. He'll never turn away an opportunity to look fancy, so he can be found in a dress shirt, slacks, and a simple dark grey vest- just nice enough not to look out of place. The kitchen seems to be covered for help - and, as mentioned, he really should steer clear - so he sticks to socializing and general clean-up, making sure no potted plants are being assaulted by coats, and after the meal, that everything is cleaned and put away. Just doing his job!

He'll probably be attached at the hip to Teddy at least half the time, and stupidly happy for all of it- moreso than someone ought to be on a simple holiday gathering, anyway. Don't be surprised if you find him smiling at nothing at all.

He also might try to bully someone into letting him have a drink, because what else are big gatherings for than breaking the rules where no one will notice?