Text is from The Liturgical Year
by Abbot Guéranger, O.S.B.
The Church enters today on the seven days which precede The Vigil of Christmas, and which are known in The Liturgy under the name of The Greater Ferias. The Ordinary of The Advent Office becomes more Solemn; The Antiphons of The Psalms, both for Lauds and The Hours of The Day, are Proper, and allude expressly to The Great Coming.
Every day, at Vespers, is sung a Solemn Antiphon, consisting of a fervent Prayer to The Messias, Whom it addresses by one of The Titles given Him in The Sacred Scriptures.
In The Roman Church, there are seven of these Antiphons, one for each of The Greater Ferias. They are commonly called The "O"s of Advent, because they all begin with that interjection. In other Churches, during The Middle Ages, two more Antiphons were added to these seven; one to Our Blessed Lady, "O Virgo Virginum"; and the other to The Angel Gabriel, "O Gabriel"; or to Saint Thomas the Apostle, whose Feast comes during The Greater Ferias; it began "O Thoma Didyme". [It is more modern than "O Gabriel"; but, dating from the 13th-Century, it was almost universally substituted for it.]
There were even Churches where twelve Great Antiphons were sung; that is, besides the nine we have just mentioned, "O Rex Pacifice" to Our Lord, "O Mundi Domina" to Our Lady, and "O Hierusalem" to The City of The People of God.
The Canonical Hour Of Vespers has been selected as the most appropriate time for this Solemn Supplication to Our Saviour, because, as The Church sings on one of her Hymns, it was in the evening of the World ("vergente mundi vespere") that The Messias came amongst us.
The Antiphons are sung at "The Magnificat", to show us that The Saviour, Whom we expect, is to come to us by Mary. They are sung twice, once before and once after the Canticle, as on Double Feasts, and this to show their great Solemnity.
In some Churches, it was formerly the practice to sing them thrice; that is, before the Canticle, before the Gloria Patri, and after the "Sicut erat". Lastly, these admirable Antiphons, which contain the whole pith of The Advent Liturgy, are accompanied by a Chant replete with melodious gravity, and by Ceremonies of great expressiveness, though, in these latter, there is no uniform practice followed.
Let us enter into the spirit of The Church; let us reflect on the great day which is coming; that, thus, we may take our share in these, the last and most earnest, solicitations of The Church, imploring her Spouse to come, to which He at length yields.