The word ‘Advent’ is from the Latin ‘Adventus,’ which means ‘coming.’ Advent is the beginning of a new liturgical year (in the Western churches), and encompasses the span of time from the fourth Sunday before Christmas, until the Nativity of Our Lord is celebrated. The first Sunday of Advent is the Sunday nearest the feast of St. Andrew the Apostle (which is November 30th), and so it will always fall somewhere between November 27th at the earliest and December 3rd at the latest. The liturgical color for this season is purple (Usually a deep purple as opposed to the lighter, red-violet shade of purple associated with Lent).
Like Lent, Advent is a preparatory season. It has significance because it is a season of looking forward and waiting for something greater; both for the annual celebration of the event of Christ’s birth, and for the time when Christ will come again.
As noted in the New Catholic Encyclopedia, during Advent, the faithful are asked:
— 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.
The exact time when the season of Advent came to be celebrated is not precisely known. Of course, it was not in practice before the celebration of the Nativity and Christmastide began; the earliest evidence shows that the feast of the Nativity of Our Lord was established within the later part of the 4th century. There are homilies from the 5th century that discuss preparation in a general sense, but do not indicate an official liturgical season. A Synod held in 590 established that Mondays, Wednesdays, and Fridays from November 11th until the Nativity would be offered according to the Lenten rite. This and other traditions, such as fasting, show that the period of time now established as the Advent season was more penitential (similar to Lent) than the liturgical season as we know it today.
A collection of homilies from Pope St. Gregory the Great (whose papacy was from 590-604) included a sermon for the second Sunday of Advent, and by 650 Spain was celebrating the Sundays (five at the time) of Advent. So it seems the liturgical season was established around the latter part of the 6th century and first half of the 7th century. For the next couple of centuries, Advent was celebrated for five Sundays; Pope Gregory VII, who was pope from 1073-85, reduced the number to four Sundays.
The themes and traditions of the Advent season have evolved throughout the history of the liturgical season. As mentioned, the early Advent season was mainly penitential, close to the theme of the Lenten season. Today a penitential theme still exists, but it is not as intense as in 7th century. Also, it is blended with the theme of prayerful, spiritual preparation for the second and final coming of the Lord, as well as the joyful preparation for the annual festive remembrance of the Incarnation and Christ’s birth.
In recent decades, a trend of wearing blue vestments rather than purple during Advent has emerged. However, purple is the appropriate vestment color, as noted in paragraph 346 of the General Instruction of the Roman Missal, in the section which discusses the prescribed colors for liturgical vestments:
Violet or purple is used in Advent and Lent. It may also be worn in Offices and Masses for the Dead.
Rose may be used, where it is the practice, on Gaudete Sunday (Third Sunday of Advent) and on Laetare Sunday (Fourth Sunday of Lent).
Advent celebration and traditions can vary from place to place, influenced by culture. However, some traditions have come to be common throughout the Catholic Church.
The Advent wreath is likely the most popular tradition, and wreaths are typically present in both the parish church and in the home. It is a more recent tradition, with the modern Advent wreath emerging in Germany and spreading throughout Europe and beyond in the 1930’s. It is often circular, representing God’s eternity, and it includes 4 candles – one for each Sunday of Advent. Many families have a wreath in the home, and will light the candles each Sunday and say Advent prayers together. A great guide for this practice is the book Season of Light.
The Jesse Tree is also a popular Advent tradition. A Jesse Tree, named for the father of David, is a tree that is decorated gradually throughout Advent with symbols or pictures of biblical persons associated with the gradual coming of the Messiah, Christ. This includes, among others, Noah, Abraham, Moses, and Joseph and Mary. The Jesse Tree tradition provides a wonderful teaching opportunity; it is a perfect way to teach and remind children about the preparatory nature of Advent.
Advent calendars are another popular tradition, although they most often follow the calendar month of December, not the four Sundays which can begin in November. Just look for Catholic Liturgical and/or Advent-themed calendars.
Source: http://www.aquinasandmore.com/catholic-articles/the-history-and-meaning-of-advent/article/173/sort/relevance/productsperpage/12/layout/grid/currentpage/1/keywords/advent – This article adapted information from The New Catholic Encyclopedia and Catholic Customs and Traditions.