This is a question often asked. I would have to invoke Professor Joad; "It depends on what you mean by Christmas". If, by Christmas you mean the celebration of the supposed birthday of Jesus Christ then no, humanists do not celebrate Christmas.

On the otherhand, if by Christmas you mean the widely accepted definition as 'a time of year', like the Winter Solstice, or Yuletide, then of course, like everybody else, we celebrate the passing of the old and the coming of the new.