If you work hard through the week, one day of that junk will not hurt you much, if at all.
Do your weights, cardio and schmoeing Monday through Friday or Saturday, and if you eat a full pizza, donuts and have 3-4 beers on Sunday your progress will not be hindered.
Your condition may suffer a bit, but your not jumping on the Olympia stage, so why not have a good shit day each week?