The hymn is a synthesis of the “O Antiphons” sung for Vespers during the octave before Christmas.
The Advent (and Christmas) hymn “Veni, Veni Emmanuel” is in fact a synthesis of the great, ancient “O Antiphons” sung for Vespers during the octave before Christmas since at least the ninth century. But while the antiphons are medieval, this hymn is way more recent: one finds it in the Psalteriolum Cantionum Catholicarum, edited in Cologne (Germany) in 1710.
The hidden riddle in “O Come, O Come, Emmanuel”
An interesting detail about this hymn is that, when the initial words of the actual “O Antiphons” are read in reverse order, they form an acrostic: take the initial letters from Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia, and you will get the Latin sentence “Ero Cras,” which can be translated as “I will be there tomorrow,” as if announcing the arrival of the Messiah.
Here, we include the translation of the first stanza, but you can look up the entire lyrics, both in Latin and English, in this link.
Veni, veni, Emmanuel
captivum solve Israel,
qui gemit in exsilio,
privatus Dei Filio.
R: Gaude! Gaude! Emmanuel,
nascetur pro te Israel!
O come, O come, Emmanuel,
and ransom captive Israel,
that morns in lonely exile here
until the Son of God appear.
R: Rejoice! Rejoice! O Israel,
to thee shall come Emmanuel!