Q message "Merry Christmas" was not intended to wish a merry Christmas to all us anon (it's only dec.12 after all).

It was an answer to the many anons saying that Christmas falls on a Monday this year, and maybe the "fateful Monday" it's on Christmas day.

Q can't say it bluntly, so he just answered "Merry Christmas", to make us understand it will be a very important day for everybody.