Here's a goody that we all see at around this time of year: "Why do atheists celebrate Christmas?"
To echo PZ, on a different subject, 'because it's fun.'
Really, do we need any other reason than that? Well here's one, if you want one:
For a couple of days, we all of us try just that little bit harder to practice what we (ahem) preach, the rest of the year 'round. We bite back on arguments and ignore perceived slights, in favour of 'having a good time if it kills us,' and of not spoiling everyone else's good time. We spend time with our families. For a fleeting day or two, we really do try our best to promote peace on earth in the most practical way we can; in our own homes.
Being the bunch of stubborn, competitive and often contrarian hairless apes that we are, the wonder isn't that we fight and argue so damned much; it's that we manage, all too briefly and incompletely, to manage it at all. If only we could stretch it a bit, to maybe New Year. If that worked we could try for Twelfth Night the next time around, and then… Yeah, I know it's naïve…
So whatever your faith, if you have one (come Pagan, Christian or Jew, as the old song says), Peace and Love, and have a good:
- A-Not-So-Nice-Day-At-Work (My sympathies; I've been there too, my friends.)
- Birthday (lest we forget those who are getting the short end of the stick, presents-wise)
- Flying Spaghetti Monster Holiday
- Just-Another-Day-I-Can't-See-What-All-The-Fuss-Is-About Grumble-Grumble
- Saint Anastasia's Day
- Summer Solstice (for those south of the tropic of Capricorn)
- Winter Solstice (for those north of the tropic of Cancer)
- Any others I might have missed
(Ho ho ho!)