I agree with the others, but in the case of your own mom, I think you will feel better if you don't just sort of shrug it off. A lot depends on how you think it made her feel not to hear from you on the actual day. Some people, like Sharnita, might not care much. To be truthful, I don't care a whole lot about birthdays, but I think I would be a little hurt, and definitely unimpressed, if my own children or my husband didn't call on my birthday. Not a big deal, but I'd care more than if it were anyone else -- even my brother or mom or best friend, I wouldn't care at all if they didn't call the right day.
So even though it is not serious, I think you'll be happiest if you apologize rather effusively and let HER tell YOU it wasn't a big deal. "oh, mom, I just realized I got all mixed up about what day it was yesterday and I completely forgot to wish you happy birthday! I'm so sorry! What must you have thought? Did you have a great day?". The point is, rather than defending yourself or giving excuses, let her be the gracious one.