I am an avowed Atheist but I love Christmas ( correction used to love Christmas before it turned in an orgy of excess, crass rampant commercialism and conspicuous consumption).
To me it is just a night excuse to be with the people I love ( Religious or not) and to celebrate, and reflect upon the year's events. I do love the traditional frippery which accompanies Christmas and love Christmas Carol concerts ( yes even as part of a service) , Christmas trees, beautiful Christmas Markets, Gluhwein, Stollen etc... and to me it is just a nice time of year if done properly.
I do not feel hypocritical as I do not pretend it has a religious meaning to me, it is just one of many cultural customs I think our society would be the poorest without. I can enjoy the ritual and the positive message behind it without feeling I have betrayed my secularism.
I will also quite Happily celebrate Diwaki, The Eid festival, Hanukkah etc... with friends from other religions.
I do not see anything wrong is sharing happiness and a positive outlook with others even if I do not have a belief in the religion.
Christmas is about being with those you love and if a message of "Peace and Good Will to all Men" is part of it then count me in.
I will either reply "Happy Christmas" or "Happy Holidays" or even "Have a wonderful Festive season" and offer it to people when out shopping etc..
The world is a place dark and depressing enough, as far as I am concerned anything even vaguely upbeat and positive cannot be that bad.
Heck even my Muslim/Sikh/Jewish friends are quite happy embracing Christmas and do not feel compromised by it.
If you are an observing Christian then its true meaning is obviously very potent , but if not it can still be a wonderful time of year.... as long as the mighty dollar does not raise its ugly head of course...