Hey, hun, slightly different situation, but I had a 1st birthday party on my EDD weekend. I also had a really big health scare related to our loss around the same time, so was very emotional.

It was weird, and hard, and I won't pretend I didn't cry, but I got there, and got through it.

My tips:
1 - decide whether or not you actually want to go. Realise that it's OK either way.
2 - think about how strong you really are. You have already survived the worst.
3 - Consider what's the worst that could happen? You end up crying and have to leave... Yeah, sucks a little bit, but nobody's going to get p'd off at you for it. And if you do, they're a waste of space anyway.
4 - talk to the party host about how they can help you. Take a deep breath and day something like "I really want to celebrate with you, but I'm finding it hard to see you since we lost Tre." (don't use euphemisms, use the baby's name, and speak frankly. This will encourage her to do the same, and WILL make things easier)
5 - continue with "so what I need from you is....." Things you might need: a room where you can escape to; the opportunity to hide out in the kitchen "helping"; for everyone to know your story so you don't have to re-tell it or face the dreaded "So, when are you having kids?" Whatever you need, tell her. Be direct.

I used to hate people seeing me cry. I still do. But one terrible day last year, when I ended up hiding in my office worrying about everyone seeing my cry, which made me cry more, I realised that I couldn't leave the building without people seeing me. So I took the deep breath, and walked into every room in the place, and said calmly "I'm going home early today. I'll be back tomorrow." And once I'd done it, it was OK. Nobody will ever think less of you for losing it or crying or needing to leave a situation.

It can be so hard, always having to take that deep breath and face things. Nobody can do it for you - although a few good friends will probably try. Hang on to them.

Good luck.