Complex sentence: It was published posthumously in Rossetti's "Poetic Works" in 1904 and became a Christmas carol after it appeared in "The English Hymnal" in 1906 with a setting by Gustav Holst.
Split into simpler sentences: It was published posthumously in Rossetti's "Poetic Works" in 1904. The poem became a Christmas carol after it appeared in "The English Hymnal" in 1906 with a setting by Gustav Holst.