There seems to be a trend with religious holidays. Colleagues come up to me and express a certain difficulty in wishing me a happy holiday given that I don't believe in the reason behind the holiday. Some do so because they think I'm Jewish, others do so because they know me as an atheist, but the result's the same.
My reply is that I assure them I am going to enjoy the upcoming holiday. What I do not usually add, given that these events tend to take place at work and work regulations prevent me from doing so while keeping my job, is the following:
I do not need an imaginary friend to enjoy a break. Especially if that friend happens to be a chauvinistic, blood thirsty ego maniac deity. I can easily rise to the occasion whether you want to call your holiday Christmas, Passover or Easter; to me they're just "that break around the end of the calendar year" and "that long weekend in April".
Doesn't sound like much, but to me that means a lot.