From Dom Guéranger's The Liturgical Year.

Regem venturum Dominum, venite, adoremus.

De Isaia Propheta.

Cap
. vi.

In anno, quo mortuus est rex Ozias, vidi Dominum sedentem super solium excelsum et elevatum: et ea quæ sub ipso erant, replebant templum. Seraphim stabant super illud: sex alæ uni, et sex alæ alteri: duabus velabant faciem ejus, et duabus velabant pedes ejus, et duabus volabant. Et clamabant alter ad alterum, et dicebant: Sanctus, sanctus, sanctus Dominus Deus exercituum: plena est omnis terra gloria ejus.
Come, let us adore the King our Lord, who is to come.

From the Prophet Isaias.

Ch. vi.

In the year that king Ozias died, I saw the Lord sitting upon a throne high and elevated: and his train filled the temple. Upon it stood the Seraphim: the one had six wings, and the other had six wings: with two they covered his face, and with two they covered his feet, and with two they flew. And they cried one to another and said: Holy, holy, holy the Lord God of hosts: all the earth is full of his glory.

Such is the glory of the Lord in the highest heavens: who could see it and live? But now, contemplate this same Lord upon our earth, during the days which have dawned upon us. The womb of a Virgin contains Him, whom heaven cannot contain. To angels His beauty is visible, but it dazzles them not; to men, it is not even visible. Not a single voice is heard saying unto Him those words of heaven: Holy, holy, holy is the Lord God of hosts! The angels no longer say of Him: All the earth is full of His glory; for the earth is witness of His abasement, and an abasement so abject and low, that the inhabitants of the earth do not even know it. At first, there was but one who knew the divine secret: the Virgin Mother; after her, Elizabeth was admitted to know that her cousin was Mother of God; and then, after the most painful and humiliating suspicions, the great mystery was revealed by an angel to Joseph. So that only three on earth know that God has come down upon it! Thus humbly did He re-enter the world, after the sin of pride had driven Him out of it. O God of the ancient Covenant, how great Thou art! and who would not tremble before Thee? O God of the new Covenant, how little Thou hast made Thyself! who would not love Thee? Heal my pride, the source of all my sins! teach me to value what Thou didst so much value. By Thy Incarnation Thou dost a second time create the world; and in this second creation, more excellent than the first, Thou workest by silence, and Thy triumph is won by self-annihilation. I wish to humble myself after Thine example, and to profit by the lessons which a God came down so low to give me. Lay low all that is high and lifted up within me, O my Jesus, for this is one of the ends of Thy coming. I abandon myself to Thee as to my sovereign Master! do with me and in me what Thou wilt.

Hymn taken from the Anthology of the Greeks
(December 23)

Antefestalia cantica Christi nativitatis mentis alacritate præcanamus; ham qui Patri et Spiritui est æqualis, per misericordiam commiserans, massam indutus luti nasci debet in Bethlehem civitate; cujus nativitatem ineffabilem pastotes eum angelis hymnificabunt.

In cymbalis resonemus, in canticis alalagmum personemus. Christi manifestatur ostensio, prophetarum finem habuerunt præconia; quem enim inter mortales dixerunt appariturum nascitur in sancta spelunca, et in præsepio reclinatur ut infans.

Bethlehem præparare; Eden, aperire; omnis terra Juda, nunc adornare, lætentur cœli, exsultent homines: in præsepio vita, in spelunca dives, advenit per misericordiæ multitudinem paupertatem Adam restaurare, absque mutatione vel confusione.

Ad te de luce vigilo, qui per misericordiam teipsum pro homine lapso exinanisti sine mutatione, et servi formam ex Virgine tulisti, Verbum Dei, pacem da mihi, Philanthrope.

Stillent ex alto aquam nebulæ: qui nubes posuit descendit ipse adorandus in nebula Virgine, ut luceat ab eo lumen inocciduum his qui antea in tenebris periculisque erant.

O dulcissimum Puerum, quomodo nutriam te? Quomodo te apprehendam, qui omnia nutu tuo tenes? Quomodo te fasciis involvam, qui omnem terram involvis nebula? clamabat sancta Domina.

Sol, fili mi, quomodo recondam te fasciis? Quomodo retinebo te aui omnia contines? Quomodo te sine metu intueri potero, quem non audent contemplari qui multos habent oculos? aiebat Christum tenens nuptinescia.

Bethlehem, adesdum, praepara quæ ad partum pertinent. I, Joseph, inscribere cum Maria; venerandum præsepium, Deiferæ fasciæ; ubi Vita involuta mortis funes disrumpet, alligans immortalitati mortales, Christus Deus noster.
Let us sing, in gladness of heart, the canticles of the pre-vigil of the birth of Christ; for he, who is coequal with the Father and the Spirit, having, in his great compassion for our miseries, clothed himself with the leaven of our clay, is to be born in the city of Bethlehem; and shepherds with angels will hymn his ineffable birth.

Let us play loud on our cymbals, let us shout our songs of victory; Christ is to appear visibly; the predictions of the prophets are fulfilled; he, who they foretold would appear amongst mortals, is to be born in a holy cave, and to lie in a crib a little child.

Get thee ready, O Bethlehem! Eden, open thy gates! Land of Juda, put on thy best! Let the heavens be glad, let men exult! To enrich the poverty of Adam by the abundance of his mercy, Life is in that crib, the rich One is in that cave, yet the divine Nature suffers no change or confusion.

From the dawn of day I watch for thee, who, in mercy for fallen man, didst empty thyself, yet still remaining God, and didst take from a Virgin the form of a servant, O thou Word of God, O Lover of men! I beseech thee, give me peace.

Let the clouds drop down dew from on high. He who puts the clouds in the air, he the adorable God, has descended in a cloud, and that cloud is the Virgin: he has done this, that light everlasting may shine from him on those who heretofore were in darkness and peril.

O most sweet Child, how shall I feed thee? said the blessed Lady. How shall I take thee into my arms, thou that holdest all things in thy power? How shall I wrap thee in swathing bands, that coverest the whole earth with clouds?

My Babe, said the Virgin Mother of Christ, how shall I hide thee, bright Sun, in swaddling clothes? How shall I so imprison thee that holdest all things? Shall I be able to fix my gaze on thee, whom the many-eyed spirits dare not look upon?

Get ready, then, O Bethlehem, all that is needed for the birth. And thou, Joseph, go and be enrolled with Mary. O crib ever venerable! O ye bands that swathe our God, holding in your folds the Life that breaks the bands of death, and ties us mortals to immortality, Christ Jesus our God.

Prayer from the Mozarabic Missal
(In the Mass of the fifth Sunday of Advent)

In proximo quidem est, Domine, dies adventus tui: sed quæsumus ut, antequam venias, expiari mereamur ab omni contagione delicti. Prius dilue, rogamus, in nobis omne quod in illa futura examinatione puniturus es; ut cum justus adveneris judex, non in nobis invenias quod condemnes.
The day of thy coming, O Lord, is near, indeed, at hand; but before thou comest we beseech thee make us worthy to be purified from every contagion of sin. First remove from us, we entreat thee, whatsoever there is in us which thou wouldst have to punish in that future examination; that so, when thou comest as our just Judge, thou mayst find nought in us to condemn.