Not speaking from experience !
As this pregnancy is my first bub, I'm not speaking from experience when I say this, I just wanted to note that before I begin !
Personally I think that while you are a major major part of your darling child's life, so are grandparents and other family members ! You can spend all day doing birthday celebrations, and then other family members can help celebrate his birthday that night while you are at the christmas party.
I don't know what it's like to feel mummy guilts, but I think that your son will have a lovely birthday even if you aren't with him the entire day. :)