Summary of comments of this office by Dom Schuster, in his work L'année liturgique.

Introduction: The Liturgy From Advent to the Epiphany

The current Roman missal begins the liturgical cycle with the first Sunday of Advent. This differs from the ancient sacramentaries, where the year opened with the solemnity of Christmas. The reason for this is that the Incarnation of the Word of God is the true center point, the millennial marker which divides the long series of centuries traversed by mankind.

It was towards the fifth century that the commemoration of the Savior's birth acquired great celebrity, and that at Ravenna, in Gaul and in Spain, there began to appear in the liturgy a special cycle of preparation for Christmas.

The holy liturgy during this period gathers from the Scriptures the most energetic passages best capable of expressing the intense desire and joy with which the holy patriarchs, the prophets and all the just of the Old Testament hastened by their desires the descent of the Son of God.

Introït

Introit: "Ad te levavi" followed by a verse of Psalm 24, well expresses the sentiments of mankind, fallen and yet full of hope, imploring the Lord, who leads them to Bethlehem by the way of truth and justice.

The Gloria is omitted at the Mass, so that it may be sung even more joyfully once again on Christmas night. At Rome, however, during the Middle Ages, the Pope would solemnly intone it on this day in the Church of St. Mary Major, a custom which fits well with the festive character of the entire office of Advent.

The Gradual, as is the rule in the ancient liturgy, is taken from the same psalm as the Introit.

The Alleluia, which originally followed the second of three readings-the Gospel was the third-is taken from Psalm 84, and expresses the desire that the Father show us henceforth His mercy and salvation: that is to say, Jesus incarnate.

In the Gospel reading of this day (Lk. 21:25-33) the Church correlates the second coming of Jesus at the end of the world in gloria majestatis suae, with His first coming at Bethlehem in humilitate passionis, in the quality of Redeemer.

The Offertory is also taken from Psalm 24, which thus is the characteristic psalm of the first Sunday of Advent, and which very well expresses the meaning: who awaits the Lord with a vigilant faith will not be confounded.

The Communion is a song of jubilation and thanksgiving, drawn from Psalm 84, as was the verse of the Alleluia. The Eucharist is the gauge of the infinite goodness of the Lord, and our land, heretofore arid and barren because of sin, now bathed by the dew of grace, will henceforth bring forth its fruit.

The collect of thanksgiving after holy Communion, the post communion, is inspired by Psalm 47. The heavenly bread in which we have participated is that which, better than any  other, will dispose us by its divine virtue to meetly prepare us for the approaching feast of our redemption.

In the lower Middle Ages, at the beginning of the new liturgical year, it was a rather widespread custom to sing, at the Introit, several verses in honor of St. Gregory the Great, the inspired editor of the Antiphonarium that bears his name, an homage, certainly merited, to the one who took care to codify and assemble the treasures of liturgical chant that we know in the Mass.