When did we start saying Merry Christmas?

"Every idiot who goes about with 'Merry Christmas' on his lips, should be boiled with his own pudding, and buried with a stake of holly through his heart."   A Christmas Carol (1843)
The exact origin of Merry Christmas as a greeting is unclear. We do know that though 'mery' has been associated with Christmas from the 1500s. In a letter to Thomas Cromwell in 1534:"And this our Lord God send you a mery Christmas, and a comfortable, to your heart’s desire."
In the following century, during the period when Cromwell's namesake and his puritan pals were in charge, the phrase was not encouraged. But its spirit, if not the exact combination of words triumphantly returned in 1775, with the first publication of the carol God Rest You Merry, Gentlemen.

Victorian Christmas

The Victorians were the first to seriously exploit the commercial opportunities offered by Christmas. Though the season did not begin until Christmas Eve, it offered what today would be called a marketing opportunity. In 1843, two seemingly unconnected events helped establish Merry Christmas as one of the most commonly used expressions in the English language. One was the publication of Charles Dickens A Christmas Carol.

These were the production of the first commercial Christmas card and the publication of Charles Dickens A civil servant Henry Cole wanted to send a Christmas message to a large number of friends and business contacts. To do this more efficiently, he designed what he called a Christmas card.
The first commercially produced Christmas card - 1843
To help cover his costs, Cole sold spare copies of his cards. These were priced at a shilling each - the equivalent of £50 today. The venture proved a commercial flop but inspired others to enter the market. In economic textbook fashion, mass production reduced costs, which in turn increased demand

 Soon the idea of exchanging cards with the slogan 'Merry Christmas' became an established Victorian 'tradition' of Christmas. In 1915 a young company named Hallmark began producing low-cost cards, further expanding the market

Ebenezer Scrooge could not stop ‘every idiot’ from using the phrase that infuriated him. ‘Merry Christmas’ not ‘Bah! Humbug' is still the universal greeting of the season.