My SO wants me to spend Christmas with her family. I have that day free, but I'm gonna say no.
Do you not get along with the family?
I'm going to do my own stuff
I get wanting to do your own thing, but isn't Christmas a time to spend together with family and friends?
They can drop dead.
Alright, that seems really cruel. If you have a problem with your partners family, you should let them know.