I agree the big meal and excess snacking once a week isn't helping FF, but I don't think you are gaining 2lbs of fat in that one day. That is like 7000 calories of excess intake, or 64 kitkats (fun size). Sucks to have the negative influence, but you giving yourself one day off a diet a week might be not be a bad thing in the long run. Just don't beat yourself up over it or try to eat perfectly on that day.