Actually, I lied. I have to correct myself. Christmas Day is over on the 25th, but Christmas is actually a season that lasts until the feast of the Baptism of Our Lord, which is the day after Epiphany, which in turn is the first Sunday after January 1st.
That could get confusing... but at least that means i can keep saying "Merry Christmas" until January 8th this year. Woot!

Last year it was longer than that.