And the version by the guy who wrote it:
I remember reading a letter to the New York Times, where a guy said he was somewhere in New York City while a bunch of carolers were flittering around and they started singing “The Christmas Song.” Some old guy came up to them and asked if he could join in, and they said “sure.”. The older people in the crowd immediately recognized the guy as Mel Torme. The carolers, who were young, had no idea who it was. So he sings in that distinctive Mel Torme voice and then goes on his way. The letter writer walked up to one of the carolers and asked if he knew that the guy they were singing with was the guy who wrote the song. The caroler said no, but offered that he had a decent voice.