why is it merry christmas instead of happy christmas

That's a great question! The term "Merry Christmas" is believed to have originated in England during the 1500s, and it's thought to be a combination of two phrases: "Merry" and "Christ's Mass." "Merry" was used as a greeting at the time, and "Christ's Mass" referred to the Christian festival of Christmas. Over time, "Merry Christmas" became a standard greeting for the holiday season. The phrase "Happy Christmas" is also commonly used in some countries, but "Merry Christmas" has become the more widely recognized greeting.