It's a female singer, and it's about how she calls to say Merry Christmas to an old boyfriend and doesn't know if she should have.
The chorus goes:
"i just called to say merry christmas, merry merry christmas.."
I always hear it on the radio, and don't know what it's called. It's an upbeat song - very bubbly and catchy. Any chance you're referring to The Carpenters "Merry Christmas, Darling?" The concept isn't quite the same. She's away from the man on Christmas, but at the end, it says "merry, merry Christmas."
Here's a link to the video:
http://www.fanpop.com/spots/christmas/vi... | 