Taken from http://www.communityofhopeinc.org/Liturgical%20Year.html#Advent
If, having described the characteristic features of Advent which distinguish it from the rest of the year, we would penetrate into the profound Mystery which occupies the mind of the Church during this season, we find that the Mystery of this Coming, or Advent, of Jesus is at once simple and threefold. It is simple for it is the one same Son of God that is coming; it is threefold because He comes at three different times and in three different ways.
'In the first coming,' says St. Bernard, 'He comes in the flesh and in weakness; in the second, He comes in spirit and power; in the third, He comes in glory and majesty; and the second coming is the means whereby we pass from the first to the third.'
This, then, is the mystery of Advent. Let us now listen to an explanation of this threefold visit of Christ, given to us by Peter of Blois, in his third sermon de Adventu: 'There are three comings of Our Lord; the first in the flesh; the second in the soul; the third at judgment. The first was at midnight according to the words of the Gospel: At Midnight there was a cry made, Lo, the Bridegroom cometh! But this first coming is long since past for Christ has been seen on the earth and has conversed among men. We are now in the second coming, provided only we are such as that He may thus come to us; for He has said that if we love Him, He will come to us and take up His abode with us. So that this second coming is full of uncertainty for us; for who, save the spirit of God, knows them that are of God? They that are raised out of themselves by the desire of heavenly things, know indeed when He comes, but whence He cometh or whither He goeth they know not. As for the third coming, it is most certain that it will be, most uncertain when it will be; for nothing is more sure than death, and nothing less sure than the hour of death. When they shall say, peace and security, says the apostle, then shall sudden destruction come upon them, as the pains upon her that is with child, and they shall not escape. So that the first coming was humble and hidden, the second is mysterious and full of love, the third will be majestic and terrible. In His first coming, Christ was judged by men unjustly; in His second, He renders us just by His grace; in His first, a lamb; in His last, a lion; in the one between the two, the tenderest of friends.'
The holy Church, therefore, during Advent, awaits in tears and with ardour the arrival of her Jesus in His first coming. For this, she borrows the fervid expressions of the prophets, to which she joins her own supplications. These longings for the Messias expressed by the Church, are not a mere commemoration of the desires of the ancient Jewish people; they have a reality and efficacy of their own, an influence in the great act of God's munificence, whereby He gave us His own Son. From all eternity, the prayers of the ancient Jewish people and the prayers of the Christian Church ascended together to the prescient hearing of God; and it was after the receiving and granting them, that He sent, in the appointed time, that blessed Dew upon the earth, which made it bud forth the Savior.
The Church aspires also to the second coming, the consequence of the first, which consists, as we have just seen, in the visit of the Bridegroom to the bride. This coming takes place, each year, at the feast of Christmas, when the new birth of the Son of God delivers the faithful from that yoke of bondage, under which the enemy would oppress them. The Church, therefore, during Advent, prays that she may be visited by Him who is her Head and her Spouse; visited in her hierarchy; visited in her members, of whom some are living, and some are dead, but may come to life again; visited, lastly, in those who are not in communion with her, and even in the very infidels, that so they may be converted to the true light, which shines even for them. The expressions of the liturgy which the Church makes use of to ask for this loving and invisible coming, are those which she employs when begging for the coming of Jesus in the flesh; for the two visits are for the same object. In vain would the Son of God have come, nineteen hundred years ago, to visit and save mankind, unless He came again for each one of us and at every moment of our lives, bringing to us and cherishing within us that supernatural life, of which He and His holy Spirit are the sole principle.
But this annual visit of the Spouse does not content the Church; she aspires after a third coming which will complete all things by opening the gates of eternity. She has caught up the last words of her Spouse, 'Surely I am coming quickly,' and she cries out to Him, 'Ah! Lord Jesus Come!' She is impatient to be loosed from her present temporal state; she longs for the number of the elect to be filled up, and to see appear, in the clouds of heaven, the sign of her Deliverer and her Spouse. Her desires, expressed by her Advent liturgy, go even as far as this: and here we have the explanation of these words of the beloved disciple in his prophecy: 'The nuptials of the Lamb are come, and His wife hath prepared herself.'
But the day of His last coming to her will be a day of terror. The Church frequently trembles at the very thought of that awful judgment, in which all mankind is to be tried. She calls it 'a day of wrath, on which, as David and the Sibyl have foretold, the world will be reduced to ashes; a day of weeping and of fear.' Not that she fears for herself, since she knows that this day will for ever secure for her the crown, as being the bride of Jesus; but her maternal heart is troubled at the thought that, on the same day, so many of her children will be on the left hand of that Judge, and havng no share with the elect, will be bound hand and foot, and cast into the darkness, where there shall be everlasting weeping and gnashing of teeth. This is the reason why the Church, in the liturgy of Advent, so frequently speaks of the coming of Christ as a terrible coming, and selects from the Scriptures those passages which are most calculated to awaken a salutary fear in the mind of such of her children as may be sleeping the sleep of sin.
This, then, is the threefold mystery of Advent. The liturgical forms in which it is embodied, are of two kinds: the one consists of prayers, passages from the Bible, and similar formulae, in all of which, words themselves are employed to convey the sentiments which we have been explaining; the other consists of extermal rites peculiar to this holy time, which by speaking to the outward senses, complete the expressiveness of the chants and words.
First of all, there is the number of the days of Advent. Forty was the number originally adopted by the Church, and it is still maintained in the Ambrosian liturgy, and in the eastern Church. If, at a later period, the Church of Rome, and those which follow her liturgy, have changed the number of days, the same idea is still expressed in the four weeks which have been substituted for the forty days. The new birth of our Redeemer takes place after four weeks, as the first nativity happened after four thousand years, according to the Hebrew and Vulgate chronology.
As in Lent, so likewise during Advent, marriage is not solemnized, lest worldly joy should distract Christians from those serious thoughts wherewith the expected coming of the sovereign Judge ought to inspire them or from that dearly cherished hope which the friends of the Bridegroom have of being soon called to the eternal nuptial-feast.
The people are forcibly reminded of the sadness which fills the heart of the Church, by the somber color of the vestments. Excepting on the feasts of the saints, purple is the color she uses; the deacon does not wear the dalmatic, nor the sub-deacon the tunic. Formerly it was the custom, in some places, to wear black vestments. This mourning of the Church shows how fully she unites herself with those true Israelites of old who, clothed in sack-cloth and ashes, waited for the Messias, and bewailed Sion that she had not her beauty, and Juda, that the sceptre had been taken from him, till He should come who was to be sent, the expectation of nations. It also signifies the works of penance, whereby she prepares for the second coming, full as it is of sweetness and mystery, which is realized in the souls of men, in proportion as they appreciate the tender love of that divine Guest, who has said: 'My delights are to be with the children of men.' It expresses, thirdly, the desolation of this bride who yearns after her Beloved, who is long a-coming. Like the turtle dove, she moans her loneliness, longing for the voice which will say to her: 'Come from Libanus, my bride! come and thou shalt be crowned. Thou has responded to my heart.'
The Church also, during Advent, excepting on the feasts of saints, suppresses the angelic canticle, Gloria in excelsis Deo, et in terra pax hominibus bonae voluntatis; for this glorious song was sung at Bethlehem over the crib of the divine Babe; the tongues of the angels are not loosened yet; the Virgin has not yet brought forth her divine Treasure; it is not yet time to sin, it is not even true to say, 'Glory be to God in the highest, and peace on earth to men of good will.'
Again, at the end of Mass, the deacon does not dismiss the assembly of the faithful by the words: Ite missa est. He substitutes the ordinary greeting: Benedicamus Domino! as though the Church feared to interrupt the prayers of the people, which could scarce be too long during these days of expectation.
In the night Office, the holy Church also suspends, on those same days, the hymn of jubilation, Te Deum laudamus. It is in deep humility that she awaits the supreme blessing which is to come to her; and, in the interval, she presumes only to ask, and entreat, and hope. But let the glorious hour come, when in the midst of darkest night the Sun of justice will suddenly rise upon the world: then indeed she will resume her hymn of thanksgiving, and all over the face of the earth the silence of midnight will be broken by this shout of enthusiasm: 'We praise Thee, O God! we acknowledge Thee to be our Lord! Thou, O Christ, art the King of glory, the everlasting Son of the Father! Thou being to deliver man didst not disdain the Virgin's womb!'
On the ferial days, the rubrics of Advent prescribe that certain prayers should be said kneeling, at the end of each canonical Hour, and that the choir should also kneel during a considerable portion of the Mass. In this respect, the usages of Advent are precisely the same as those of Lent.
But there is one feature which distinguishes Advent most markedy from Lent: the word of gladness, the joyful Alleluia, is not interrupted during Advent, except once or twice during the ferial Office. It is sung in the Masses of the four Sundays, and vividly contrasts with the somber color of the vestments. On one of these Sundays, the third, the prohibition of using the organ is removed, and we are gladdened by the grand notes, and rose-colored vestments may be used instead of the purple. These vestiges of joy, thus blended with the holy mournfulness of the Church, tell us, in a most expressive way, that though she unites with the ancient people of God (thus paying the debt which the entire human race owes to the justice and mercy of God), she does not forget that the Emmanuel is already come to her, that He is in her, and that even before she has opened her lips to ask Him to save her, she has already been redeemed and pre-destined to an eternal union with Him. This is the reason why the Alleluia accompanies even her sighs, and why she seems to be at once joyous and sad, waiting for the coming of that holy night which will be brighter to her than the most sunny of days, and on which her joy will expel all her sorrow.
Text courtesy of Apologia