In the December 2004 / January 2005 issue of Free Inquiry, Tom Flynn explains his own position:
And what did you do for Ramadan this year?" That's the question I often pose when fellow humanists ask why I boycott Christmas.
Unless my interlocutor is of Muslim heritage, has close Muslim friends, or is an avocational multiculturalist, the is almost invariably, "Um, nothing."
"Interesting." I reply. "Why not?"
"Well, why would I observe Ramadan? That's not my religion."
"Exactly. That's why I don't celebrate Christmas."
For what it's worth, 2004 marks the twentieth anniversary of my personal decision to cease and desist from almost everyone's favorite holiday. After all, it's not the birthday of anyone I know -- and, if more secular humanists (and members of other cognitive minorities) would make themselves more visibly "Yule free," it might go far to rebut the harmful social fiction that American society is in some sense foundationally Christian. For two decades now, I have rejected Christmas altogether, from hot buttered rum to "rum-pa-pum-pum."
There are good arguments for both sides of the issue. As noted above, the Christian character of Christmas is diminishing over time and the ancient pagan aspects are taking on ever greater importance. Christians don't consider themselves pagans, yet have no problem putting up a tree and mistletoe. Atheists and humanists don't consider themselves pagan and also don't mind putting up a tree or mistletoe. If a few Christian aspects of the day are included, does it really matter very much?
Perhaps it does, or at least it can for some people. It's reasonable to argue that celebrating Christmas helps further the idea that America is a Christian Nation. At the same time, though, does not celebrating it do more to undermine this claim than celebrating it in an openly secular, non-Christian manner? If the decision between celebrating vs. not celebrating is to turn on political considerations, there's a lot to be said for helping along the process of secularization and de-Christianization.
Of course, it's also legitimate to question whether this should be a political question at all. Not celebrating Christmas because it's not part of your religion and/or because you don't care about it makes more sense, to me at least, than doing so in order to make a political statement. Tom Flynn mentions both reasons but I would place more weight on the first -- the second, in my opinion, should be regarded as simply an added benefit rather than an actual motivation. Christians have begun to use Christmas to make political statements; humanists and atheists shouldn't imitate them.