If I was you I would start the optifast tommorrow and IF I feel mega mega vile monday then just drop it and wait until after the show and dinner. If you just feel hungry and not great give it a go and have the day off for the show and dinner. the reason I am saying do it like this is that it wont undo everything for 1 bad day, BUT if you are somebody who sufferes terribly when starting optifast mega headaches feels like you want to throw up etc etc then seriously dont put yourself though that twice.
applesauce