why do we say merry christmas instead of happy christmas

That's a great question! The phrase "Merry Christmas" has been used for centuries, and there are many theories about its origin. Some believe that the word "merry" was used to describe the joy and happiness associated with the holiday, while others think it was meant to wish someone good health and prosperity. Whatever its origin, "Merry Christmas" is a time-honored tradition that has become a part of our culture and is now used to express our best wishes to others during the holiday season.