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 alms deeds as a preparation for the advent of the Lord, and on one Sunday (the second) from St. Jerome’s commentary on Isaiah 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 inglory; 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.
from Catholic Encyclopedia