Merry today, happy tomorrow would be pretty much being merry always.
Slip in Christmas, and New Year, and it reads Merry Christmas, Happy New Year; or Happy Christmas, and Merry New Year, which is the most conventional form of Christmas greeting.
That is pretty much saying:
“Rejoice always, and again I will say, rejoice!”
We don’t always realize that, do we?
Merry Christmas, and Happy New Year 2018.