Julie:
What are you bringing to the party? A pie or dessert or something? Maybe that is what you should be thinking about instead of trying on every top you have...
Besides if you started now, you'd be late for the party by time you tried them all on

Casual means without effort. When it's time to get ready just open up your closet and grab the closest slacks and top that match and go with it! Then accessorize appropriately. A little makeup and you're out the door. Don't forget the dessert!
They are already expecting to see you, the real you, so don't worry about it! Are you afraid you'll pick out something too butch? Fat chance! Besides, after tomorrow there will be lots of times you can show off that wonderful wardrobe of yours to them.
Relax, or do I have to come over there and get Zen on your butt?
You're going to have a fine time! Enjoy it!
-Sandy