why do we say merry christmas and not happy christmas

That's a great question! The tradition of saying "Merry Christmas" is believed to have originated in the 16th century when the word "merry" was used to describe a cheerful or pleasant disposition. Over time, it became a common greeting during the Christmas season, and it has since become a standard phrase used to wish people well during the holidays. While "Happy Christmas" is also a common greeting, "Merry Christmas" has become the more popular phrase due to its historical and cultural significance.