We Wish You a Merry Christmas is a popular Christmas song that originated in London, England.
Several carolers, as well as the Brown family, sang the song during December 25, 1845. The carolers also answered Doc Brown's questions, about what had happened to his wife and children, to the tune of the song (for example, "The lady she was arrested, for Tannen her mettle tested, his hand on her chin it rested, so she punched his ticket!") When Marty finally asked the group a question, he added "And lose the tune!"; they answered without singing.