Best friend's birthday party

So it's my best friend's birthday party today and she's one of those people who can eat whatever they like and still stay skinny (and I mean reeeally skinny) so there'll more than likely be pizza and cake and biscuits and all that stuff.

Pizza I'm not worried about because I don't really like it much, but cake and biscuit calories add up sooo fast.

I normally eat about 1200 calories a day (occassionally 1300) and have been recovering from a 2 year ED for almost 2 months.

I've had about 400cals so far today (including a small banana and apple) so that gives me 800 for the party which I know will disappear very quickly and I'll have practically no idea how many calories will be in the food I eat.

My main question is can I treat today as a 'cheat day' (within reason, obviously I won't be stuffing myself stupid) and eat a bit more than normal or will this cause me to gain loads because I'm in recovery?

A lot of people say that a cheat day can sometimes help shock your metabolism.  If you've been eating healthy and working out a lot of people reward themselves w/ 1 big meal.  I'd probably half 1/2 a slice of cake instead of the full.  and maybe just don't butter the biscuits.  If you have pizza you can make sure to blot all the grease off of it and don't eat all the crust.  If you eat slower you'll fill up much faster and just drink lots of water instead of filling up on soda.  This way you still are able to feel like you're part of the party and can enjoy everything that everyone else is eating and not feel guilty in the morning.  Just dont hop on the scale first thing.  Give your body a few days to balance.   You could always have a healthy snack before the party too so you're not as hungry.   Good luck!  Cake is always tempting.