The order of the antiphons isn't accidental. If we work backwards, beginning with the last title and take the first letter of each antiphon — Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia — the Latin words ero cras are formed, meaning, “Tomorrow, I will come.” The Lord Jesus, whose coming we have prepared for in Advent and to whom we refer in these seven Messianic titles, tells us: “Tomorrow, I will come.”
There is, however, another antiphon which is firmly part of our Patrimony.
Of course, most of the Catholic Church already shares our Patrimony’s gift regarding the O Antiphons in the metrical translation of these antiphons, the universally beloved: “O Come, O Come Emmanuel.” That translation is, in large part, the work of the famed Anglican priest, translator and hymnographer, John Mason Neale (1818-1866), to whose scholarly and literary gifts the Anglican Church owes its recovery of the great treasury of pre-Reformation Latin hymnody.
But regarding the antiphons themselves, check any of the Latin originals or Anglo-Catholic liturgical revival English translations of the venerable Sarum Use Missals and Breviaries and you may be surprised to see, in the Kalendar of these volumes, the notation O Sapientia (the first of the O Antiphons) opposite December 16 rather than December 17, which is a clear indication that in the Sarum Use, the O Antiphons began a day earlier than they did in the Roman Rite. This is because there was an extra O Antiphon proper to the Sarum Use.
Sarum began the O Antiphons with O Sapientia on December 16th because on December 23, as the Roman Rite was completing its cycle of the O Antiphons by singing its seventh one, “O Emmanuel,” the ancestors of the Anglican Patrimony, having sung “O Emmanuel” the day before, December 22nd, were completing their O Antiphons by singing their unique eighth O Antiphon — a most fitting antiphon indeed to echo throughout the monasteries and churches of the land known then – and now again – as “Our Lady’s Dowry,” the antiphon O Virgo virginum:
O Virgo virginum, quomodo fiet istud? quia nec primam similem visa es, nec habere sequentem. Filiae Jerusalem, quid me admiramini? Divinum est mysterium hoc quod cernitis.This changes the acrostic from "Ero cras" [Tomorrow, I will come] to "Vero cras" [Truly, tomorrow].
O Virgin of virgins, how shall this be? for neither before thee was any like thee, nor shall there be after. Daughters of Jerusalem, why marvel ye at me? the thing which ye behold, is a divine mystery.