The rationale for the phrase is that the word "Happy" is frequently used around the new year, holidays, birthdays, and anniversaries. However, around Christmas, they use 'Merry' instead of 'Happy.' The reason why Christmas is greeted with 'Merry Christmas' rather than 'Happy Christmas' stems from a centuries-old proverb.
Generally, 'Merry Christmas' conveys an emotional and uncontrolled connotation of celebration, whereas 'Happy Christmas' conveys a conservative and repressed meaning of celebration.
The Queen of England bids everyone a "Merry Christmas." Every year on Christmas Day, Queen Elizabeth II of England greets the people of the United Kingdom with the words 'Happy Christmas.' Rumor has it that she favours the word 'happy' to'merry' because the latter connotes boisterousness and drunkenness.
Furthermore, historians think that previous church leaders in the United Kingdom encouraged Christian followers to be 'happy' rather than'merry.' The explanation for this is based on what the language means: 'happy' is a feeling, whereas'merry' is a behaviour. Furthermore, in a letter to Henry VIII's senior minister Thomas Cromwell, the bishop John Fisher wished "Merry Christmas." The letter was written in London in at least 1534.