I really sympathize with you. I appreciate that it's 'just one day', as so many others have said, but when you've had your own family traditions for years, then suddenly it all changes, it can make you feel sad.
Even for those who are not religious, Christmas can still be a very important occasion for many of us, for when families get together. When family dynamics change, it can be hard to get used to new ways. I'm in a different situation to you, but this is going to be our first Christmas ever, since my own children were born, that my family aren't all going to be together, and, whilst I knew it would happen one day, it still upsets me. I guess it's just something that we have to get used to.
You have the birth of your grandchild to look forward to, so try to focus on that, and congratulations on being a grandmother-to-be!