As many people know, but many forget, Christmas isn't just one day, but a twelve day event. The "twelve days Christmas" aren't a countdown, but the days between Christmas itself and Epiphany. It's not just the schools that make Christmas into a multi-week party, it's always been that way.
So Merry Christmas! And tomorrow, and tomorrow, and tomorrow. You should be able to say it until January 6 guilt-free!