Ah, sorry - I'd lost track of dates, this is for the birthday party Italian restaurant dinner.
If it were me, I'd eat a normal-sized breakfast and lunch. If you have ~800 calories early in the day, you've still got room for ~500 calories of dinner, which is a reasonable-sized dinner. Plus you won't be starving, so won't be as tempted to go overboard. Order something that you'll enjoy, even if it isn't the absolutely-lowest-calorie choice available. Eat small bites, chew slowly, savor what you're eating. If you're going to use a big chunk of calories on something special, get every bit of enjoyment you can out of those calories, rather than feeling guilty, because guilt is going to make that food taste like cardboard, and cardboard is not worth the calories. Take a sip of water between bites. Pay attention to your body, and stop eating when you're satisfied (which will happen before you're completely full). Take the rest home if you don't want to waste it, or leave it there, or let someone else finish it. If everyone else is getting dessert, and you want dessert, order something you'll enjoy, and do the same thing - small bites, savor your food, stop when you're satisfied but not completely full.
If they have steak, and you like steak, a grilled steak that you trim the visible fat from is usually a relatively low-calorie choice. (Says I, who loves steak.)
I personally will have a bite or two of something unhealthy that I really like, because I don't like having a "I can't eat that" mentality, and because I know I can stop at a bite or two. (There are some days, with some things, that I know I'm going to mindlessly snarf, so I don't have even a bite or two.) I do salad with dressing on the side, and do the "dip your fork, then stab the salad" thing, to save on dressing calories so I can use those calories for something I like better. (Vinagrette is usually 2 parts oil to one part vinegar, so a tablespoon has 2 teaspoons of fat.) I'm not a huge fan of bread, so I'd swap my bread calories for polenta calories if it were me.
Worst case scenario, you have, what, 1,000 calories for dinner. Puts you at maintenance calories for the day, which makes it a good day.