The answer to that is easy; tell them you are not celebrating Christmas but the winter solstice and the birth of the Sun God.
The early Christians high jacked this Pagan festival and claimed it as the birth of Christ the Son of God..... Of course they have really no idea when he was actually born, day month or year for that matter. Best guess is either March or October depending on which researcher you believe.
During the celebration the people would brring a live evergreen tree into their dwelling and decorate with the gree leaves and berries from mistletoe and holly.
They would also exchange gifts and presents.... sound familiar ?
For what it's worth they also high jacked the Pagan festival of spring, where we celebrate the Goddess of Fertility, Ester... hence all the eggs and fluffy bunnies.... again nothing to do with anybody getting nailed to anything. Although it was quite a wild celebration so lots of people would have got hammered....!
It's like so many things to do with the any nd all religions, the deeper you dig backwards you will find they invented nothing.... just took the bits they wanted and tried very hard to destroy the rest.
They are still trying !