Advent marks the beginning of the Christmas season and the Church year for most Western churches. The word "Advent" means "arrival" or "coming" in Latin and represents the approach of Christ's birth (and fulfillment of the prophecies about that event) and the awaiting of Christ's second coming. It is composed of the four Sundays before Christmas day, starting on the Sunday closest to November 30th, which is the feast day of St. Andrew the Apostle, and ending on Christmas. Because Christmas is on a different day from year to year, Advent may last anywhere from 22 to 28 days.
In the 4th and 5th centuries, Advent was the preparation for the "Epiphany" rather than Christmas. (Epiphany is celebrated in early January and focuses on various events in Jesus' life such as the visits of the magi, His baptism and miracles.) It was also a time for new Christians to be baptized and welcomed into the church, while members of the church examined their hearts and focused on penance. Religious leaders exhorted the people to prepare for the feast of Christmas by fasting. Some say that early documents show that those leaders treated Advent as a second Lent.
Sometime in 6th century Rome, the focus of Advent shifted to the second coming of Christ. In the 9th century, Pope St. Nicholas reduced the duration of Advent from six weeks to the four weeks we currently observe. And finally, sometime in the middle ages--approximately the 1500's--an additional focus on the anticipation before Christ's birth was added to that of His second coming. For a more in-depth perspective, visit The History of Advent.