Written on Christmas Eve, huh? Seems appropriate.
The story I read somewhere a long time ago was that the organ in the church in a place called Oberndorf, Austria, was out of commission at Christmas, 1818. The priest, Joseph Mohr, was inspired by a recent visit he had made to a family with a newborn child. He wrote a poem (I guess that's what you'd call it), and the church organist, Franz Gruber, set it to music and played it on guitar at Midnight Mass.
I won't vouch for the truth or accuracy of that story, but it does lead me to one of my all-time favorite Christmas memories. I forget if it was my first or second Christmas in Philadelphia, but it happened at the staff Christmas party at the historical society, where I was working at the time. The book conservator was a sort-of unreconstructed hippie, with shoulder-length hair gone to salt-and-pepper, a similar mustache and goatee, and a baritone voice (yeah, I found him quite sexy). He also played guitar. He brought his guitar to the party, and he played
Silent Night while a bunch of us stood around him and sang it. Considering that the hymn was actually written for guitar, I got goosebumps while we were singing it.