Why do we say "Merry Christmas" and yet so rarely use the word merry at other times of the year? I got curious and did a little research. Merry seems to have been popularized by Charles Dickens in A Christmas Carol, published in 1843, the same year as the first commercial Christmas card, which also uses the phrase. "Merry Christmas" appears as early as 1565, but doesn't seem to have gained currency until the 1840s, when Queen Victoria popularized several of the rituals we now think of as commonplace at Christmas: decorating a tree and sending cards to family and friends. It was Prince Albert who brought the traditions from Germany, and desire to emulate the royal family spread the traditions throughout the world.
Ironically, the modern-day English usually wish each other a happy Christmas, rather than a merry one. This may be attributed to the fact that "merry" retains some connotation of drunkenness in the UK, which perhaps was lost in the more puritanical eras of US history.
But the most interesting aspect of merry's history has nothing to do with Christmas at all. Merry is a palimpsest of a word, dating back to the most distant roots of European languages. Merry's ancestry includes the word murgijaz, a Proto-Germanic word meaning "short-lasting," and the Proto-Indo-European root mreghu- which simply meant "short." It is suggested that the connection to pleasure comes from the notion of "making time fly" — that time feels short in a pleasurable state. So within merry, the ideas of enjoyment and evanescence find themselves inextricably linked. To wish someone a "Merry Christmas," in the very old sense, is to wish them pleasure and to express a hope that they'll savor that pleasure acutely before it passes.
All of which seems a perfect sentiment for a holiday that comes once a year. I hope you're enjoying the time with friends and family, and send wishes for lots of joy. Merry Christmas, or Merry whatever-you-celebrate!