According to present  usage, Advent is a period beginning with the Sunday nearest to the feast of St. Andrew the Apostle (30 November) and embracing four Sundays. The first Sunday may be as early as 27 November, and then Advent has twenty-eight days, or as late as 3 December, giving the season only twenty-one days.
With Advent the ecclesiastical year begins in the Western churches. During this time the faithful are admonished
to prepare themselves worthily to celebrate the anniversary of the Lord’s coming into the world as the incarnate God of love,
thus to make their souls fitting abodes for the Redeemer coming in Holy Communion and through grace, and
thereby to make themselves ready for His final coming as judge, at death and at the end of the world.
To attain this object the Church has arranged the Liturgy for this season. In the official prayer, the Breviary, she calls upon her ministers, in the Invitatory for Matins, to adore “the Lord the King that is to come”, “the Lord already near”, “Him Whose glory will be seen on the morrow”. As Lessons for the first Nocturn she prescribes chapters from the prophet Isaias, who speaks in scathing terms of the ingratitude of the house of Israel, the chosen children who had forsaken and forgotten their Father; who tells of the Man of Sorrows stricken for the sins of His people; who describes accurately the passion and death of the coming Saviour and His final glory; who announces the gathering of the Gentiles to the Holy Hill. In the second Nocturn the Lessons on three Sundays are taken from the eighth homily of Pope St. Leo (440-461) on fasting and almsdeeds as a preparation for the advent of the Lord, and on one Sunday (the second) from St. Jerome’s commentary on Isaias 11:1, which text he interprets of the Blessed Virgin Mary as “the rod out of the root of Jesse”. In the hymns of the season we find praise for the coming of Christ, the Creator of the universe, as Redeemer, combined with prayer to the coming judge of the world to protect us from the enemy. Similar ideas are expressed in the antiphons for the Magnificat on the last seven days before the Vigil of the Nativity. In them, the Church calls on the Divine Wisdom to teach us the way of prudence; on the Key of David to free us from bondage; on the Rising Sun to illuminate us sitting in darkness and the shadow of death, etc. In the Masses the intention of the Church is shown in the choice of the Epistles and Gospels. In the Epistle she exhorts the faithful that, since the Redeemer is nearer, they should cast aside the works of darkness and put on the armour of light; should walk honestly, as in the day, and put on the Lord Jesus Christ; she shows that the nations are called to praise the name of the Lord; she asks them to rejoice in the nearness of the Lord, so that the price of God, which surpasses all understanding, may keep their hearts and minds in Christ Jesus; she admonishes them not to pass judgment, for the Lord, when He comes, will manifest the secrets hidden in hearts. In the Gospels the Church speaks of the Lord coming in glory; of Him in, and through, Whom the prophecies are being fulfilled; of the Eternal walking in the midst of the Jews; of the voice in the desert, “Prepare ye the way of the Lord”. The Church in her Liturgy takes us in spirit back to the time before the incarnation of the Son of God, as though it were really yet to take place. Cardinal Wiseman says:
We are not dryly exhorted to profit by that blessed event, but we are daily made to sigh with the Fathers of old, “Send down the dew, ye heavens, from above, and let the clouds rain the Just One: let the earth be opened, and bud forth the Redeemer.” The Collects on three of the four Sundays of that season begin with the words, “Lord, raise up thy power and come” — as though we feared our iniquities would prevent His being born.
DURATION AND RITUAL
On every day of Advent the Office and Mass of the Sunday or Feria must be said, or at least a Commemoration must be made of them, no matter what grade of feast occurs. In the Divine Office the Te Deum, the joyful hymn of praise and thanksgiving, is omitted; in the Mass the Gloria in excelsis is not said. TheAlleluia, however, is retained. During this time the solemnization of matrimony (Nuptial Mass and Benediction) cannot take place; which prohibition binds to the feast of Epiphany inclusively. The celebrant and sacred ministers use violet vestments. The deacon and subdeacon at Mass, in place of the dalmatics commonly used, wear folded chasubles. The subdeacon removes his during the reading of the Epistle, and the deacon exchanges his for another, or for a wider stole, worn over the left shoulder during the time between the singing of the Gospel and the Communion. An exception is made for the third Sunday (GaudeteSunday), on which the vestments may be rose-coloured, or richer violet ones; the sacred ministers may on this Sunday wear dalmatics, which may also be used on the Vigil of the Nativity, even if it be the fourth Sunday of Advent. Pope Innocent III (1198-1216) states that black was the colour to be used during Advent, but violet had already come into use for this season at the end of the thirteenth century. Binterim says that there was also a law that pictures should be covered during Advent. Flowers and relics of Saints are not to be placed on the altars during the Office and Masses of this time, except on the third Sunday; and the same prohibition and exception exist in regard to the use of the organ. The popular idea that the four weeks of Advent symbolize the four thousand years of darkness in which the world was enveloped before the coming of Christ finds no confirmation in the Liturgy.
It cannot be determined with any degree of certainty when the celebration of Advent was first introduced into the Church. The preparation for the feast of the Nativity of Our Lord was not held before the feast itself existed, and of this we find no evidence before the end of the fourth century, when, according to Duchesne [Christian Worship (London, 1904), 260], it was celebrated throughout the whole Church, by some on 25 December, by others on 6 January. Of such a preparation we read in the Acts of a synod held at Saragossa in 380, whose fourth canon prescribes that from the seventeenth of December to the feast of the Epiphany no one should be permitted to absent himself from church. We have two homilies of St. Maximus, Bishop of Turin (415-466), entitled “In Adventu Domini”, but he makes no reference to a special time. The title may be the addition of a copyist. There are some homilies extant, most likely of St. Caesarius, Bishop of Arles (502-542), in which we find mention of a preparation before the birthday of Christ; still, to judge from the context, no general law on the matter seems then to have been in existence. A synod held (581) at Mâcon, in Gaul, by its ninth canon orders that from the eleventh of November to the Nativity the Sacrifice be offered according to the Lenten rite on Monday, Wednesday, and Friday of the week. The Gelasian Sacramentary notes five Sundays for the season; these five were reduced to four by Pope St. Gregory VII (1073-85). The collection of homilies of St. Gregory the Great (590-604) begins with a sermon for the second Sunday of Advent. In 650 Advent was celebrated in Spain with five Sundays. Several synods had made laws about fasting to be observed during this time, some beginning with the eleventh of November, others the fifteenth, and others as early as the autumnal equinox. Other synods forbade the celebration of matrimony. In the Greek Church we find no documents for the observance of Advent earlier than the eighth century. St. Theodore the Studite (d. 826), who speaks of the feasts and fasts commonly celebrated by the Greeks, makes no mention of this season. In the eighth century we find it observed not as a liturgical celebration, but as a time of fast and abstinence, from 15 November to the Nativity, which, according to Goar, was later reduced to seven days. But a council of the Ruthenians (1720) ordered the fast according to the old rule from the fifteenth of November. This is the rule with at least some of the Greeks. Similarly, the Ambrosian and the Mozarabic rites have no special liturgy for Advent, but only the fast.
From the Catholic Encyclopedia
In Christ Crucified and the Most Victorious Heart of Jesus.