The following is taken from The Liturgical Year.
'At length,' says St. Peter Damian, in his Sermon for this holy Eve, 'at length we have got from the stormy sea into the tranquil port; hitherto it was the promise, now it is the prize; hitherto labour, now rest; hitherto despair, now hope; hitherto the way, now our home. The heralds of the divine promise came to us; but they gave us nothing but rich promises. Hence, our Psalmist himself grew wearied, 'and slept, and, with a seeming reproachful tone, thus sings his lamentation to God: "But thou hast rejected and despised us; thou hast deferred the coming of thy Christ." [Ps. lxxxviii.]. At another time he assumes a tone of demand, and thus prays: "O thou that sittest upon the Cherubim, show thyself!" [Ibid. lxxix.]. Seated on thy high throne, with myriads of adoring Angels around thee, look down upon the children of men, who are victims of that sin, which was committed indeed by Adam, but permitted by thy justice. Remember what my substance is [Ibid. lxxxviii.]; thou didst make it to the likeness of thine own; for though every living man is vanity, yet inasmuch as he is made to thy Image, he is not a passing vanity [Ibid. xxxviii.]. Bend thy heavens and come down, and turn the eyes of thy mercy upon us thy miserable suppliants, and forget us not unto the end!
'Isaias, also, in the vehemence of his desire, thus spoke: "For Sion's sake I will not hold my peace, and for the sake of Jerusalem I will not rest, till her Just One come forth as brightness. Oh! that thou wouldst rend the heavens, and wouldst come down!" So, too, all the Prophets, tired of the long delay of the Coming, have prayed to thee, now with supplication, now with lamentation, and now with cries of impatience. We have listened to these their prayers; we have made use of them as our own, and now, nothing can give us joy or gladness, till our Saviour come to us, and, kissing us with the kiss of his lips, say to us: "I have heard and granted your prayers."
'But, what is this that has been said to us: "Sanctify yourselves, ye children of Israel, and be ready; for on the morrow, the Lord will come down"? We are, then, but one half day and night from the grand visit, the admirable Birth of the Infant-God! Hurry on your course, ye fleeting hours, that we may the sooner see the Son of God in his crib, and pay our homage to this world-saving Birth. You, Brethren, are the Children of Israel, that are sanctified, and cleansed from every defilement of soul and body, ready, by your earnest devotion, for to-morrow's mysteries. Such, indeed, you are, if I may judge from the manner in which you have spent these sacred days of preparation for the Coming of your Saviour.
'But if, notwithstanding all your care, some drops of the stream of this life's frailties are still on your hearts, wipe them away and cover them with the snow-white robe of Confession. This I can promise you from the mercy of the divine Infant: he that shall confess his sins and be sorry for them, shall have born within him the Light of the World; the darkness that deceived him, shall be dispelled; and he shall enjoy the brightness of the true Light. For how can mercy be denied to the miserable this night, in which the merciful and compassionate Lord is so mercifully born? Therefore, drive away from you all haughty looks, and idle words, and unjust works; let your loins be girt, and your feet walk in the right paths; and then come, and accuse the Lord, if this night he rend not the heavens, and come down to you, and throw all your sins into the depths of the sea.'
This holy Eve is, indeed, a day of grace and hope, and we ought to spend it in spiritual joy. The Church, contrary to her general practice, prescribes, that if Christmas Eve fall on a Sunday, the fasting alone should be anticipated on the Saturday; but that the Office and Mass of the Vigil should take precedence of the Office and Mass of the fourth Sunday of Advent. How solemn, then, in the eyes of the Church, are these few hours which separate us from the great Feast! On all other Feasts, no matter how great they may be, the solemnity begins with first Vespers, and until then the Church restrains her joy, and celebrates the Divine Office and Sacrifice according to the Lenten rite. Christmas, on the contrary, seems to begin with the Vigil; and one would suppose that this morning's Lauds were the opening of the Feast; for the solemn intonation of this portion of the Office is that of a Double, and the Antiphons are sung before and after each Psalm or Canticle. The purple Vestments are used at the Mass, but all the genuflexions peculiar to the Advent Ferias are omitted; and only one Collect is said, instead of three, which always denote that the Mass is not that of a solemnity.[*] Let us enter into the spirit of the Church, and prepare ourselves, in all the joy of our hearts, to meet the Saviour who is coming to us. Let us observe with strictness the fast which is prescribed; it will enable our bodies to aid the promptness of our spirit. Let us delight in the thought, that before we again lay down to rest, we shall have seen Him born, in the solemn mid-night, who comes to give light to every creature. For, surely, it is the duty of every faithful this happy Night, when, in spite of all the coldness of devotion, the whole universe keeps up its watch for the arrival of its Saviour. It is one of the last vestiges of the piety of ancient days, and God forbid it should ever be effaced!
[*Maestro's note: The three collects are actually not said in the 1962 missal, but only one is prayed - a common difference between 1962 and pre-1962 missals, and a topic of liturgical interest in itself.]