This one is known as a much loved carol but the poet Christina Rossetti wrote it as a poem in response to a request for a Christmas poem. It was published posthumously in Rosetti's Poetic Works and became a carol after it appeared in 1906. It has two well-known settings but I think I prefer the Harold Darke version (2nd video below).
King's College choir singing Gustav Holst's version
Harold Darke's version