Happy Christmas is my favorite, I'll admit. Here's the 1773 version.
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Don Foster's "Author Unknown" 2000
Of all the poems and letters and magazine articles that Henry Livingston wrote during his eighty years on the planet, the earliest to survive begins with the greeting "A happy Christmas to my dear Sally Welles." "A Visit from St. Nicholas" ends with the greeting "Happy Christmas to all." One might guess that a "happy" Christmas, which today sounds quite ordinary, was as commonplace in Livingston's time as a "merry" Christmas, but the guess would be wrong. Literature Online, for example, locates the earliest "Happy Christmas" in 1823 in a little poem beginning "'Twas the night before Christmas..." A broader survey of English and American literature, from 1390 ("murie Cristes masse") through the Christmas of 1823, shows that "Merry Christmas" was commonplace and "Happy Christmas" rare. Charles Fenno Hoffman, who ascribed the poem to Moore, changed "Christmas" to "New Year" at lines 1 and 56. Other editors changed the last line of "A Visit" to read "Merry Christmas to all..." Many later editors followed suit, as if "Happy Christmas" were a mistake. But a "Happy Christmas!" sounded just fine to Henry Livingston.
Whether Clement Moore preferred a "Merry Christmas" or a "Happy Christmas" cannot be determined. From what I've seen of his extant writing in verse and prose, including personal correspondence, Moore never said "Happy Christmas" (or "Merry Christmas") to anyone.