Parties are always hard...!
Personally, I've tried different ways of dealing with it; last week seems to have been the most successful so far.
What I did was, I ate dinner before going to the party, but not a huuuge meal (and I ate relatively light during the day). So then I knew I could afford to have a small amount of snacks at the party. I knew I was going to be going over my calorie range for the day, but didn't want to go too overboard.
I also drank diet cider, and not much of it.
Before this, I had tried the tactic of eating a big meal before a party in the hopes that if I was full, I wouldn't snack at the party... that didn't go well! haha. I always ended up snacking anyway.
It's hard to stay on track at parties, but try not to go overboard and you should be ok. Most importantly, enjoy yourself and try not to stress out about it too much!!
I hope this post helps you a little!