Ok, try to put yourself into this situation:
You have no idea when your birthday is (for whatever reasons, be creative) ... no one will tell you (including parents, etc.) and your relatives pretend you don't exist unless you live with them.... then one day one of your aunt calls you up and says: "I know your birthday isn't until tomorrow but I won't be able to reach you tomorrow so I wanted to say happy birthday today"
What would you do and how would you feel ?