What is the biggest selling number one Christmas single?

The biggest-selling number-one Christmas single is "Do They Know It's Christmas?" by Band Aid. Released in 1984, the song was written by Bob Geldof and Midge Ure and performed by a collective of British and Irish pop stars, including Bono, George Michael, Sting, and Annie Lennox. The song was released in aid of famine relief in Ethiopia and sold over 11 million copies worldwide, becoming the biggest-selling single of all time at the time.