The Magnificat antiphons at Vespers for 17 to 23 December inclusive, the Greater Ferias of Advent, are known as the “Great Antiphons” or the “O Antiphons” (as they all start with “O”). Unlike throughout the year, these Magnificat antiphons are sung standing. Each antiphon invokes the Messias under a different title: Sapientia (Wisdom), Adonaï (Lord), Radix Jesse (Root of Jesse), Clavis David (Key of David), Oriens (Orient / Day-spring), Rex Gentium (King of the Nations / King of the Gentiles), and lastly Emmanuel (Emmanuel / God is with us). The first letter of each of these titles can be read as a backwards acrostic which spell, in Latin, “Ero cras,” which means “tomorrow I will be [there].” These Antiphons are the basis of the hymn “O Come, O Come Emmanuel.”
(See also Ecclus. 24:5 and Wis 8:1 for the source material of this O Antiphon.)
