when do you say merry christmas

Well, it depends on what part of the world you're in. In some places, people say "Merry Christmas" as soon as the clock strikes midnight on Christmas Eve, while others wait until Christmas Day morning. In some countries, they even have different greetings altogether, like "Joyeux Noel" in France or "Buon Natale" in Italy. But no matter how or when you say it, the important thing is that it comes from the heart and that you share it with those you care about.