"Merry Christmas"

[Barbara Need]

>a correspondent asks, in a message with the header "Happy Christmas":
>-----
>Why do USAns usually say '"merry" Christmas'?
>
>Okay, not "why", but where did the tradition start?
>------
>
>i'm away from my dictionary trove, but a quick on-line search nets me
>nothing, though i suspect the OED has the goods on this.
>
>arnold

I was recently rereading an Agatha Christie in which someone wishes
another person "Merry Christmas", so the question may be, when the
British start saying "Happy Christmas"?

Barbara