How do you make your mom come home to her surprise birthday party on time without looking suspicioius?

Convince her that there is a serious problem with her house and that she needs to see it RIGHT away!

If the party isn't at her house, you could tell her a friend of hers has something to celebrate at their home, a restaurant, or wherever, but wants to keep it a secret so she can tell your mother herself when she arrives.

Be sure to tell the friend, and other friends (who are presumably guests at the party) what you're intending to tell your mother in case she rings around to try to find out about the 'secret'.

Don't even hint at anything which might cause your mother to worry! Stress it's something she'll be pleased about.