What is the origin and history behind the season of Advent?

The word ‘Advent’ is from the Latin word ‘Adventus,’ which means coming. Advent marks the beginning of a new liturgical year and encompasses the span of time from the fourth Sunday before Christmas, until the Nativity of Our Lard 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.
 
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 of Our Lord was established within the later part of the 4th century. There are homilies from the 5th century that discuss preparation in the general sense, but do not indicate an official liturgical season. A collection of homilies from Pope St. Gregory the Great (whose papacy was from 590-604) included a sermon from 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 1073-1085, reduced the number to four Sundays.