Christmas is being celebrated since 4th century AD, and the usage of greeting cards in the celebration came into existence not before 1565, when it appeared in The Hereford Municipal Manuscript: “And thus I comytt you to God, who send you a mery Christmas.” “Merry Christmas and a Happy New Year” (thus incorporating two greetings) was in an informal letter written by an English admiral in 1699. The same phrase is contained in the 16th century secular English carol “We Wish You a Merry Christmas,” and the first commercial Christmas card, produced in England in 1843.
Word Merry had started to have a meaning which was ” Cheerful, outgoing and Jolly, when in 1843, Charles Dickens’ A Christmas Carol was published. This new Meaning of the word Merry became prominent in his vook ” A Christmas Carol”.
The instant popularity of A Christmas Carol, the Victorian era Christmas traditions it typifies, and the term’s new meaning appearing in the book popularized the phrase “Merry Christmas.
However being used from 19th Century there are still parts of the world which heavily used ” Happy Christmas” instead of Merry like UK and Ireland.
One reason may be the Methodist Victorian middle-class influence in attempting to separate wholesome celebration of the Christmas season from common lower-class public insobriety and associated asocial behaviour, at a time when merry also meant “tipsy” or “drunk”. Queen Elizabeth II is said to prefer “Happy Christmas” for this reason.
In the American poet Clement Moore’s “A Visit from St. Nicholas” (1823), the final line, originally written as “Happy Christmas to all, and to all a good night”, has been changed in many later editions to “Merry Christmas to all”, perhaps indicating the relative popularity of the phrases in the USA.