The distinction between the two is rooted in tradition and connotation:
"We wish you a Merry Christmas and a Happy New Year!" Simple: "Wishing you magic, love, and joy this Christmas!" P6T6H6C - Merry Christmas and Happy New Year 20...
(Christmas) often describes a short-term, cheerful, and noisy celebration. The distinction between the two is rooted in
Why can we say “Happy New Year”, but not “Merry ... - Facebook P6T6H6C - Merry Christmas and Happy New Year 20...
Queen Elizabeth II famously preferred "Happy Christmas," as "merry" was historically associated with the rowdy behavior of commoners.