I agree with the comments on orthography - nitpicking on international forums is not good netiquette. Do join the American English/British English debate
Back on topic - I think the terms merry Christmas
, happy Christmas
, and happy birthday
are common collocations born out of usage and many languages also have their regular expressions - another reason by the way, of being careful if using machine translations.
The two adjectives have slightly different meanings.merry adj. ['mɛriː] merrier ◊ merriest
Joyful or jovial.
make merry v.expr. [meɪk 'mɛriː]
To be jovial; to indulge in hilarity; to feast with mirth.
happy adj. ['hæpiː] happier ◊ happiest
1. Enjoying or showing or marked by joy or pleasure or good fortune: “a happy smile”: “spent many happy days on the beach”: “a happy marriage.” 2. Experiencing pleasure or joy: “happy you are here”; Synonyms: pleased. 3. Well expressed and to the point: “a happy turn of phrase”Courtesy of Ultralingua Dictionaries