But...Christmas is December 25th. Why would you say "merry Christmas" any other day but on Christmas? Surely, non-Christians won't be celebrating Christmas, so they can't have a merry or un-merry Christmas. They just might be taking advantage of the sales.
Like I say, on the other 364 days of the year, tell folks to have a nice day. On Christmas, say "merry Christmas." If you want to be a real pain in the neck, wish people a prayerful Advent from now until December 25. Merry Christmas from December 25th to January 5th (these are the 12 days of Christmas called Christmastide). Onl January 6th, you can say "Happy Epiphany!"
It isn't Christmas folks. It's Advent until December 25th.