"May your sweater be ugly and your hot chocolate be spiked."

Queen Elizabeth II famously preferred "Happy Christmas," as "merry" was historically associated with the rowdy behavior of commoners.

The distinction between the two is rooted in tradition and connotation:

(New Year) is seen as a calmer, steadier feeling suitable for fresh starts and long-term goals throughout the entire year.

Why can we say “Happy New Year”, but not “Merry ... - Facebook

(Christmas) often describes a short-term, cheerful, and noisy celebration.