Latin: "Veni veni Emmanuel, Captivum solve Israel...." English: "O come, o come, Emmanuel, And ransom captive Israel." The advent of Emmanuel the savior, descendant of David, is requested, and people are told to celebrate his coming
Although I know of no field collections which include this song, it seems to me that it is now widely enough sung that it belongs in the Ballad Index.
Certainly it is *old* enough. Johnson claims the words come from the seventh century. This is probably too early (my guess is that that's based on theories about the history of Latin hymnwriting). But the whole is found in the French National Library manuscript (Bibliotheque Nationale) fonds latin MS 10581.
Manuscripts of this era are very difficult to date; book hands hardly changed from the twelfth to fifteenth centuries. The _New Oxford Book of Carols_ proposes a thirteenth century date. We can at least say that it is from the fifteenth century or earlier, with the text very likely older.
The standard translation is by J. M. Neale, who also gave us the much weaker "Good King Wenceslas." The _New Oxford Book of Carols_ gives an alternate translation (termed a revision) by T. A. Lacey. It appears, at first glance, a more accurate translation -- but distinctly worse as poetry (e.g. the last line of the first Latin stanza is "privatus Dei Filio," loosely, "deprived of the Son of God." Neale butchers this as "until the Son of God appear," but at least gets an easy-to-sing line. Lacey produced "far from the face of God's dear son").
"Emmanuel" ("God With Us") refers back to Isaiah 7:14, where Isaiah prophecies that the threat to Judah from Israel and Damascus shall ease before the new-born child Immanuel (as it is properly transliterated from the Hebrew) reaches the age of having moral sense.
This prophecy is picked up in Matthew 1:23, which uses the Greek spelling "Emmanuel" (which worked its way into Latin and hence into the song). There is rather a curiosity here, in that Matthew normally translates the Hebrew himself, but in this particular version cites the previous Septuagint translation, which has in fact a mistranslation (Septuagint and Matthew read "a virgin shall bear a son," but the Hebrew reads "a young woman shall bear a son"). Clearly this ties in somehow with the Matthean doctrine of the Virgin Birth (which is found in full form only in Matthew; while Luke calls Mary a virgin at the time of her betrothal, he doesn't say that Joseph didn't touch her after that).
Several verses of the song refer to Emmanuel as a descendent of David. This does not come from Isaiah; again, it's Matthew who provides the link, giving a genealogy of Jesus going back to David (Matt. 1:2-16, though Matthew's genealogy omits several names known from the Book of Kings, plus it is at least six or seven generations too short to bring us from the Exile to the time of Jesus).
All of this is somewhat reinforced by Luke. Luke never mentions Emmanuel, but he does have a genealogy linking Jesus to David (Luke 3:23-38), though it differs from Matthew's in irreconcilable ways. (Not that it matters. It was a thousand years from David to Jesus. By the time Jesus was born, everyone in Judea was descended from David, though not necessarily in the male line). Luke also provides much of the imagery of celebration at the arrival of the Messiah (see chapter 2). - RBW