God Rest Ye Merry, Gentlemen
"God Rest Ye Merry, Gentlemen" was an ancient Earth Christmas carol. It was this song that was being played by a group of robotic Santas before they attacked Rose Tyler and Mickey Smith on Earth. (TV: The Christmas Invasion) The Santas also played it just before abducting Donna Noble in a taxi. (TV: The Runaway Bride)
A group of children were singing this song when the Tenth Doctor arrived in London on Christmas Eve 1851, just before his encounter with the Cybermen. (TV: The Next Doctor)
The song was also playing before Wilfred Mott entered a church commemorating "the Sainted Physician" where he first met the woman. It was also played by a brass band shortly before the Tenth Doctor regenerated. (TV: The End of Time)
The melody of the carol was the same as that of the "Venusian lullaby" occasionally used by the Third Doctor to lull his foes into a state of tranquility. (TV: The Dæmons, The Curse of Peladon, PROSE: Venusian Lullaby)
Professor Capra sang a version of "God Rest Ye Merry, Gentlemen" to himself. (AUDIO: Flip-Flop)