I'd probably go with an open mind and hope that you have lucked out and gotten a "normal" BM. There are plenty of them out there. It's just that most of us are on this site because the BM in our case is a nutburger.

Please don't think that all BMs are crazy. Many are capable of having a normal relationship - yes there will be ups and downs but it does not have to be all out war.

Trust your gut. I hope you do get a normal one.

Maybe I'm just biased because of our situation, but is the joint celebration for the child the normal thing?

Do they do joint Christmas, Halloween, Easter or anything or is it only birthdays?

I ask because I personally think that while yes, it's nice for kids to see their parents still able to be civil, that once you divorce having mom and dad together for these 'family' events sends the wrong message to the kids.

Of course I don't know anything about the current tenor of the interactions between your bf and his ex, but I'd wonder what the expectation around this stuff would be moving forward.

And yes, only go if You want to. You didn't have a child with the woman, just because she wants something does Not mean you have to oblige her. And any guff about it "being best for the child" shouldn't sway you either. Do what makes you comfortable. If you do otherwise at some point it is likely that you will start to resent the mom, the dad and the kid.

Well, my BM instead of meeting me like a normal person, found out from the skid where my exH worked, tracked him down, and asked HIM about me. My exH is a pathalogical liar-BM didn't know that then, but she does now. I lived under the same roof with her kid for 9 freakin' months before she met me face to face.
I like what my DH did. Like a real man, we set up a time and went to my ex"s house and they met like that. Because I have a BD with my ex, so it would be weird if my BD lived in the same house as a man my ex had never met.
I know in my case, I was afraid BM was going to be one of those beautiful perfect polished people and I was going to feel ugly. I know that sounds shallow, but I'm just being honest. I was pleased when I finally did meet her to see that she had completely let herself go. I don't respect her as a mother either because she sucks as a mom in so many ways.
The best advice I can give you is this-call her up and see if she wants to have coffee or lunch. BEFORE the party.
Also, this dual parent holiday crap...before my ex and I were each in our own serious relationship it worked, but out of respect for the BF/GF/spouses, not such a great idea. For the kid, it's great, but I dont know, maybe you will actually get a good BM who doesn't exist do meddle and bother you.
I'm one of those BM's so they do exist. I really totally try to stay out of my ex's life. I only speak to him about our Bd, and I try to reach him at work so his wife doesn't have to sit there getting pissed because he's on the phone with me.

I would go and meet her, just act friendly and see what happens! You could be one of the lucky ones where people actually do get along for the kids. I can tell you that my ex and dh get along. We now spend holidays together with the kids are are planning a trip together next year--me and dh, my ex and his wife, our kids and other extended family.

Just curious, what brought you to this site in the first place?

In your situation I would go, be friendly and cordial, but not overly so. You may be one of the few that has a normal BM (like other posters said). If BM starts to act up or try to intentionally make you uncomfortable, you have a couple choices:

1) Get up and leave. Excuse yourself politely and be sure you drive your own car there so you have a way to get out.
2) Ignore her. Be sure to not sit next to her or too close to her.
3) Kill her with kindness and let her make herself look like the jealous psycho she may be.

DH and I don't do any joint events with BM. Ever.

After divorcing, my ex and I NEVER did joint birthdays or holidays. We might be present at the same school event, but did not sit together. We had a normal relationship of communication for the children's scheduling, etc. Whether this is too much closeness with the ex is something for you to consider if you do marry.

I attempted to meet my DH's ex before I even met the SD. But she wouldn't agree to a time or place. I just wanted to get off on the right foot, so she could see I was a normal person, who wouldn't harm her child. Little did I know she was a psychobitch. That was revealed in small bits, over time.

I had no problems with the woman my ex married, she was normal.

It's a crapshoot. I hope you get a good one.