From Prof. Geoffrey Pullum on Language Log (one of my favorite blogs):

For about thirty years, Professor Laurie Taylor (retired from the University of York) has been doing a humor column in Times Higher Education, a U.K. university administration magazine, in the form of a newsletter from an imaginary Poppleton University. This week it included a painfully awkward message from an equally imaginary Interfaith Chaplain, struggling to find some kind of contentful and seasonal greeting that couldn't possibly offend anyone of any faith:

You know, very soon we will be reaching that special time of the year when people who subscribe to certain religious beliefs rather than to others will be celebrating what they regard as a very significant event. May I therefore take this opportunity to wish all such believers a very happy special time of the year...

Language Log, however, is not quite so inclined to imagine that simple words of greeting will shock or disgust anyone; it seems to us that such worries are rather closely related to word taboo, with which we have little sympathy. So it has been our custom for some years to come out quite boldly and use the C word at this season. We love writing for you, and as time permits, in our odd moments of spare time between full-time university jobs or research projects, we will continue to do so. And whatever your religion or lack of it, we wish you a happy Christmas Day.

Hear, hear!