Congratulations on Williams! It's an incredibly well-respected program that will set you up for success––however which way you define that––upon graduation.
RE protocol: I'm not sure there is one, but celebrate and publicize as you see fit. If this is a big deal to you then scream it from the rooftops and post it everywhere, if that's what you want to do! A few of my own friends have done just that, while others have only told their closest friends; it's completely a personal decision.