As others have said, congrats and have fun! To which I add some advice:
You are going to be the last band at a party. If you are nervous now, you will be even more nervous sitting through the first two bands waiting for your turn to play. The temptation will be to have a few drinks to calm your nerves. If you really want to play well, resist this temptation and go on sober.
Remember, most people don't go to a party hoping to have a lousy time; they're going to be predisposed to like you. And by the time you go on, they'll probably be drunk enough that they won't notice any clams anyway. Just do what you rehearsed, and don't make faces if you make a mistake.
Before you know it, this moment you're so nervous about will be nothing but a happy memory, and an experience you'll be eager to repeat.
Now go forth and rock!