I "bank" some weight loss beforehand in maintenance stage. During weight loss I would put in some good exercise in the day or so before...and plan on exercising big time the next day. My goal was to reduce the damage as quickly as possible by exercising extra.
On the day itself I will be light on breakfast and lunch. Have no or little snacks during the day. If it is a late eating time start I will have an apple etc at about the time I would normally eat (I generally like to eat dinner early).
At the venue, I would resist having any entrees and will not touch the bread rolls (and butter). I would select the nicest safest option I can find. You should be able to find something that you will like that isn't going to blow you out of the water. The great unknown is serve size. This is where you will have to be careful and disciplined. As soon as it comes decide what is a proper potion size and divide your plate accordingly. Once you have done that, eat what is yours and when you are finished put a paper napkin or the like on your plate, set your knife and fork in the "surrender position" and move the plate clearly away from you straight away. Drinks are often a big problem at dinners. I just stick to water and the coffee after the dinner. Dessert is treated like the main meal. Immediately potion controlled and then got rid off.
I went to a swish dinner last Wednesday for my wife's work at Government House. Prepared in advance and ready for any trouble. It was a set menu and a late eating time but I was organised. However when the food finally was put before us - the serve size was so meagre that I need not have worried. No one was going to gain weight on the amount they were feeding us!