D.A.
My step-son's birthday is Nov 28th...since he turned 6 we have had his friend's party 2 weeks before his birthday. He doesn't mind as long as he gets a party. Most kids won't mind either as long as they know they're getting one. We just explained to my son that more of his friends will be able to come if it isn't close to his actual birthday due to many going out of town. He's been perfectly fine. He is 9 1/2 now.
Don't make it a big deal as to it not being "on" their birthday and neither will they. Even if it's after their birthday.
Hope this helps.