Did you know that what some consider to be the oldest Advent hymn is O Come, O Come Emmanuel? The hymn dates back to the ninth century, and is based on the O antiphons of Advent, which, in some traditions, are sung on the third Sunday in Advent.
Here's another interesting tidbit about O Come, O Come Emmanuel: Even though it is considered the oldest Advent hymn, it was not translated into English until the nineteenth century by a Church of England clergyperson, John Mason Neale. Last night I heard a quartet sing the hymn and the last stanza they sung was not familiar to me or to our Lutheran hymnals. However, upon further investigation, I discovered that it was part of the Mason Neale translation.