Advent is the 4 week period before Christmas when the Church celebrates the first coming of Christ and anticipates his second coming. Advent can fall on any date between (and including) November 27 and December 3.
The word "advent," from the Latin adventus (Greek parousia), means "coming" or "arrival." The season of Advent is focused on the "coming" of Jesus as Messiah (Christ or King). Our worship, scripture readings, and prayers not only prepare us spiritually for Christmas (his first coming), but also for his eventual second coming. This is why the Scripture readings during Advent include both Old Testament passages related to the expected Messiah, and New Testament passages concerning Jesus' second coming as judge of all people. Also, passages about John the Baptist, the precursor who prepared the way for the Messiah, are read. All of these themes are present in Catholic worship during Advent, which The Catechism succinctly describes:
When 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. By celebrating the precursor's birth and martyrdom, the Church unites herself to his desire: "He must increase, but I must decrease" (524).
Since Advent looks forward to Christ's birth and Incarnation, it is an appropriate way to begin the Church Year. However, Advent is not part of the Christmas season itself, but a preparation for it. Thus, Catholics do not sing Christmas hymns, or use Christmas readings, in Mass until December 25th, the first day of the Christmas season.
The liturgical color for Advent is violet (except for the Third Week of Advent, often called Gaudete Sunday, in which rose may be used), and the season is somewhat penitential, similar to Lent, although not so explicit and emphatic. The character of worship during Advent is more solemn, quiet, and less festive than during other times of the year. In the Catholic Church, for example, the Gloria in Excelsis is not used. The use of violet reflects the general themes of Advent: penitence (generally expressed more in terms of expectant hope) and royalty. Some prominent feasts fall within the Season of Advent, including the Feast of the Immaculate Conception and the Feast of Our Lady of Guadalupe.
Secular culture and many non-Catholic churches celebrate the day of Christmas, but take it outside of the context of Advent and Christmastide. However, Christmas is not meant to be an isolated day, but a festival of the Incarnation in the midst of the Church year. Christmas is only properly understood after having the preparation provided by Advent. In the midst of the secular excesses leading up to Christmas, Advent provides a welcome solace and an opportunity to continually re-orient ourselves to God's will as we expectantly wait with patriarchs, prophets, and kings for the true meaning of Christmas: the Incarnation of God the Son.
The New Testament identifies Jesus as the expected Jewish Messiah, although Jesus was not the Messiah most Jews at the time expected, a warrior who would forcibly overthrow the Romans. The gospel writers are clear that Jesus did not come to establish an earthly kingdom, or deliver the Jewish people from the Romans, but rather he proclaimed a heavenly kingdom available to Jew and Gentile alike. Even though early Christians understood that Jesus reigned in the Church, they knew that all things had not been subjected fully to him, so Christians understood that there existed a future finalization of his kingdom (see Catechism 680). Thus, early Christians eagerly awaited the return of Jesus in glory "to achieve the definitive triumph of good over evil," when he would judge the living and the dead (Catechism 681, 682). These prominent Scriptural themes form the basis of our Advent season.
The first clear reference to a celebration of Advent occurs in the 6th century. Prior to this time, there were celebrations and fasts resembling our current Advent season. St. Hilary of Poitiers (d. AD 367) and the Spanish Council of Saragossa (AD 380) spoke of a three week fast before Epiphany. Pope St. Leo the Great preached many homilies about "the fast of the tenth month (i.e. December)" prior to Christmas. The Gelasian Sacramentary (AD 750) provided liturgical material for the five Sundays before Christmas as well as Wednesdays and Fridays. The Western Church eventually settled on 4 Sundays of Advent, which has the season beginning at the very end of November or the very beginning of December, starting immediately after Ordinary Time. Until the 12th century, in many geographical areas, Advent had a more festive tone, and white vestments were still occasionally used. However, Advent became more closely related to Lent as Christ's second coming became more and more a prominent Advent theme, as especially seen in the seventh century Bobbio Missal. Advent proper is unknown in the East, although the Eastern Churches have a long fast before Christmas. This fast lasts longer than the Western Advent season and begins in mid-November. Advent, or the Eastern equivalent fast, is celebrated in all Catholic and Orthodox Churches.
During the Reformation, many Protestants attacked or de-emphasized many Christian holy days and seasons, disconnecting Protestantism from the rhythms of the Church Year. However, some Reformation churches, like the Anglicans, retained Advent. Possibly because of the liturgical movement or maybe as a reaction to the excesses of secular Christmas values, celebrating Advent has become more popular in non-Catholic and non-Orthodox churches. Lutherans, Anglicans, Methodists, Presbyterians, and even many evangelical groups have incorporated Advent into their worship service to varying degrees. However, many Protestant churches have fallen short of celebrating the true meaning of Advent, treating the season as more of an early extension of Christmas. As non-Catholics and non-Orthodox begin to rediscover the Church year, Advent (like Lent) is one of the first results, flowing naturally from churches looking to fill in the gaps around Christmas and Easter.