I'm not going to a surprise party for a friend if its held on my birthday
Why wouldn't you go to the surprise party?
I don't want to ruin that surprise for him. I mean if I show up he will know I've attended so this party
If your reason is not to ruin the surprise then that's okay. You can go after they surprise him.
Still I don't think I should go there.
Well you don't have to show up if you don't want to. But I think it'd be nice if you made an appearance at some point.