See also: our list of 'Christmas Card Sayings and Expressions'.
The traditional greeting at Christmas - very commonly used on Christmas cards.
"A Merry Christmas and a Happy New Year to You" was the verse that was shown on the first commercially available Christmas card in 1843. Christmases has been merry long before that though. The use of 'Merry Christmas' as a seasonal salutation dates back to at least 1534, when, on 22nd December, John Fisher wished the season's greetings in a letter to Thomas Cromwell, recorded in Strype Ecclesiastical memorials, 1816):
And this our Lord God send you a mery Christmas, and a comfortable, to your heart’s desire.
1843 was the date of the publication of Charles Dickens' Christmas Carol and it was around that time, in the early part of the reign of Queen Victoria, that Christmas as we now know it was largely invented. The word merry was then beginning to take on its current meaning of 'jovial, and outgoing' (and, let's face it, probably mildly intoxicated). Prior to that, in the times when other 'merry' phrases were coined, for example, make merry (circa 1300), Merry England (circa 1400) and the merry month of May (1560s), merry had a different meaning, that is, 'pleasant, peaceful and agreeable'.
That change in meaning is apparently viewed with disfavour by Queen Elizabeth II, who wishes her subjects a 'happy' rather than 'merry' Christmas in her annual Christmas broadcasts. The idea of a modern-day merry England is presumably unwelcome at the palace.
The best-known allusion to merriment at Christmas is the English carol God Rest Ye Merry, Gentlemen. The source of this piece isn't known. It was first published in William Sandys' Christmas Carols Ancient and Modern in 1833, although versions of it probably existed as a folk-song and tune well before that but weren't written down. Sir Thomas Elyot, lists the phrase 'rest you merry' in his Dictionary in 1548:
"Aye, bee thou gladde: or joyfull, as the vulgare people saie Reste you mery."
It is often assumed that the carol's lyric portrays the wish that jovial gentlemen might enjoy repose and tranquility. The punctuation of the song suggests otherwise though - it's 'God rest ye merry, gentlemen', not 'God rest ye, merry gentlemen'. In this context 'to rest' doesn't mean 'to repose' but 'to keep, or remain as you are' - like the 'rest' in 'rest assured'.
'Rest ye merry' means 'remain peacefully content' and the carol contains the wish that God should grant that favour to gentlemen (gentlewomen were presumably busy in the kitchen). It isn't the 'rest' that is being given but the 'merry'. Anyone misreading that comma is in good company though. God Rest Ye Merry, Gentlemen was the carol that Dickens was referring to in A Christmas Carol:
"The owner of one scant young nose, gnawed and mumbled by the hungry cold as bones are gnawed by dogs, stooped down at Scrooge's keyhole to regale him with a Christmas carol: but at the first sound of
God bless you, merry gentleman!
May nothing you dismay!
Scrooge seized the ruler with such energy of action that the singer fled in terror."