To be honest I always say happy Christmas when I greet people at Christmas time, but when I use both It's always merry Christmas and happy new year. I think it comes from the song which has buried itself in the English-speaking psyche. 'We wish you a merry Christmas We wish you a merry Christmas We wish you a merry Christmas And a happy new year' etc. By the way, er... Happy new year!

It's all about collocation. 'Happy' often goes with 'Christmas' and 'Merry' often goes with 'New Year'. If you switched them around they would not be wrong. It just wouldn't sound correct. In fact in a movie called 'Coming to America,' Eddie Murphy, an African prince who is new to America, shouts 'Merry New Year!' This scene is funny because it sounds strange.

who says you can't say it like this? ;)

So far in my knowledge, it's not incorrect, so – as the nice panda above said – why not? Honestly I prefer Happy Christmas – for John Lennon's beautiful song, you know; on the other hand "merry" is a very pretty word, I think, for it sounds a bit ancient, oldish kind: it's more stressed before New Year than before Christmas (the last one's so common... we don't even notice it)... so let's go for your new formula!

Why not? You can be a non-conformist. :)