I do agree CHRISTMAS is a great holyday to celebrate and you and you non-religious relatives are more than welcome to join us and participate!
What were they celebrating over 2000 years ago on December 25th?
Well, there was a pagan festival at that time as well as a roman festival called Saturnalia that was close to that time. And there are many Gods who were supposedly born of virgins on December 25 including Horus, Krishna, Dionysus.
Also, I wanted to direct you to this article on wikipedia (i know, not the greatest source), but it talks about the origins of using Xmas rather than Christmas. It's not intended to be a slight against Christians/Christianity.
http://en.wikipedia.org/wiki/Xmas#Use_of_.22X.22_for_.22Christ.22
I'm not trying to make this a thread about arguing religion though, just some food for thought here.