Gotcha! So for me, if you don't know yet either if she's coming or not, you could say

I wish Mary would come to the party on Saturday...(you don't know yet).

However, you someone tells you that she wont be able to come and you know for a fact that she's not coming it sounds a little weird.

Ex:

-Did you invite Mary to the party?

-Yes, I wish she would come...

-I just spoke with Mary and she wont be able to come to the party this Saturday.

-Really? That's bad, I wish she could come.

Again, if you say, "I wish she would come..." is understandable too...it just sounds a little odd to me.

Sid.