What is Advent?
Pope Pius XII, in his 1947 encyclical on the liturgy Mediator Dei, wrote that during Advent, "the Church arouses in us the consciousness of the sins we have had the misfortune to commit, and urges us, by restraining our desires and practicing voluntary mortification of the body, to recollect ourselves in meditation, and experience a longing desire to return to God who alone can free us by His grace from the stain of sin and from its evil consequences." (n. 154)
The word "advent" comes from the Latin adventus, which means "an arrival; a coming to". The verb is advenire ("to arrive, to come to"): ad- ("towards") + venire ("to come"). The season of Advent is the time when we celebrate the coming of the Messiah: not only making present his first coming (the Incarnation and then the Nativity of our Lord Jesus Christ) but also anticipating his second coming (his glorious and triumphant return at the end of the world).
But whenever there's an arrival, there's also... a wait. Advent is a season of waiting, of expectation. We are awaiting Christmas, the Nativity of the Lord. Now the secular world is telling us that it’s already Christmas, but the Church in her wisdom knows better than the secular world. We celebrate this season for only a few weeks, but the world was in the season of Advent for thousands of years.
The nation of Israel, the whole world, the whole universe was awaiting the coming of the Messiah, the Christos, the Christ. He came into the world through the mystery of the Incarnation, by which God the Son, the Word, the second Person of the Most Holy Trinity, manifested Himself on earth in a body of flesh and blood, Who "by the Holy Spirit was incarnate of the Virgin Mary, and became man." (Nicene Creed, new English translation)
The Incarnation inaugurated the last nine months of that wait… and at the end of those nine months, all creation came to adore the Christ who had finally arrived: the poor, the rich, shepherds, Magi, sheep, angels, even the very stars themselves. This is why during Advent, we don’t sing the Gloria: we are anticipating the first singing of the Gloria by the angels to the shepherds of Bethlehem.
Here is an excerpt from Pope Benedict's homily at first vespers on the First Sunday of Advent last year:
Advent is, par excellence, the season of hope. Every year this basic spiritual attitude is reawakened in the hearts of Christians, who, while they prepare to celebrate the great Feast of Christ the Savior's Birth, revive the expectation of his glorious second coming at the end of time. ...As the Catechism (#524) explains, "[w]hen the Church celebrates the liturgy of Advent each year, she makes present this ancient expectancy of the Messiah, for by sharing in the long preparation for the Savior's first coming, the faithful renew their ardent desire for his second coming. (cf. Rev. 22:17)"
[T]he celebration of Advent is the answer of the Church-Bride to the ever new initiative of God the Bridegroom, "who is and who was and who is to come" (Rev. 1:8). God offers to humanity, which no longer has time for him, further time, a new space in which to withdraw into itself in order to set out anew on a journey to rediscover the meaning of hope.
Even the norms for the Roman liturgy seek to express the character of the season: "During Advent the floral decoration of the altar should be marked by a moderation suited to the character of this season, without expressing prematurely the full joy of the Nativity of the Lord. ... In Advent the organ and other musical instruments should be used with a moderation that is consistent with the season's character and does not anticipate the full joy of the Nativity of the Lord." (GIRM 305, 313)
What does Advent mean for you?
Update: Why are rose vestments (permitted to be) worn on the Third Sunday of Advent? I'll let Father Z answer that:
Easy: Rose is the color used on the fourth Sunday of Lent!The reason violet (or "purple" in our parlance) vestments are worn during Advent is because it is a penitential season and a time of preparation; they are also a sign of Christ's royalty. The color is perhaps evocative of the sky before dawn (and rose, then, gives a glimmer of the sunrise).
In Rome for centuries now there are celebrations of Mass during the great seasons of Lent/Easter and Advent/Christmas at "station" churches. In Lent, the fourth Sunday is called "Laetare" (which means in Latin pretty much what "Gaudete" means…"rejoice!"). The station Mass for "Laetare" Sunday was at the Basilica of the Holy Cross in Jerusalem not far from the Lateran Basilica (the Pope’s cathedral in Rome).
It was the custom on this day, stretching perhaps back to the time of Pope St. Gregory III (740), for the Pope to bless special roses made of gold that were to be sent to the Catholic kings, queens and notables. Thus it was called Dominica de rosa.... Sunday of the Rose.
It doesn’t take much imagination to develop rose vestments from this custom.
Soon the practice of using rose (the technical term for the color to be used is rosacea... from the Latin adjective for "made of roses") spread from that basilica to the rest of the City. As a Roman practice it became part and parcel of the Roman Missal promulgated by Pius V through the world.