How to prepare for a cheat day?

I've been doing strict carnivore for 2 weeks now. Mostly grass fed burger patties, some chicken and salami (pork). No eggs, no dairy. No spices except salt. Only 2 black coffees a day. 3 glasses of diet coke in 2 weeks. I'm at my lowest weight now since February 2013. I'll participate in a birthday party at a burger/pizza/smoked barbeque restaurant 2 weeks from now, that will be my cheat day. I'll have some fries, mushrooms, sauce, fried onions, maybe a drink. How should I prepare my stomach/guts for this? Should I eat some soup/veggies the day before? I don't want pain/diarrhea. I will continue carnivore the next day, without a question