When the Holidays come around I try to use the term Happy Christmas! It's done for a couple of reasons, but mostly because the term Merry means agreeable or pleasant. Christmas is more than that. From a Catholic point of view, the Nativity is a happy moment.
I'm not going into a large treatment of the philosophy behind happiness, but suffice to say that it is a far superior greeting than merry. We don't say Merry Easter, or Merry Feast of Christ the King, but we do call my birth month the Merry, Merry month of May.....
So, when you hear me say or see me write Happy Christmas, it's because I understand the importance of happiness, from a theological and philosophical point of view. If you would like a more complete understanding, I would suggest starting with this and then move forward from there.
As Midnight Mass approaches, I'll simply say this and I mean it to each and every person I come across....Happy Christmas!