After Thanksgiving, I say "Have a Merry Christmas," if know the person is a Christian, practicing or not. If their Jewish, I say, "enjoy the holidays." From Christmas Eve through the day before New Years Eve, I say Merry Christmas; Happy New Year thereafter, thoughout most of January.
Although admittedly, I'm not following it, Orthodox should be celebrating Christmas on Christmas Day, for the 12 consecutive days, greeting eachother with "Christ is in our midst," with a reply, "He Is and always shall be." I've noticed Orthodox Slavs practice this tradition, Greeks, at least in America, don't currently.