When I respond “Thank you. Merry Christmas,” I sometimes sense that I have actually created some tension. While many of those to whom I wish “Merry Christmas” may actually be happy that ...

Let's look at wish first of all. In your 'Merry Christmas' example, or when you wish someone good luck or Happy Birthday, you are expressing the hope that they will have good luck in the future ...

I wish you all who celebrate the best possible Christmas. And I wish the best to those of you who celebrate something else.