Maybe they are both expected to be there at a mutual friend's birthday.

If that person is a good friend, ask her/him if you can pass to avoid your ex? Or ask if she/he knows if she is coming? Seems like the right thing to do, especially if you really feel that way. And yes, I don't blame you for feeling that way, even though what I said may be mean. It's just me being odd me (which hasn't helped me).

It seems clear meeting again will be both painful and unhelpful to both of you. So I guess follow their advice.