I'm going to have to miss Christmas with my family, sadly.
Why are you not spending this Christmas with your family?
I have some obligations that I have to meet on that day.
It's understandable to obligations that need to be fulfilled, but its good to put your family first. They may end up feeling like you can't be there for them during special occassions.
What can I do? I have to be elsewhere too at times.
Your family is always going to be there though. You should be too. Whatever it is that you have other obligations to, can't it be arranged some other way?