For denominations that celebrate Advent, it marks the beginning of the church year.
In Western Christianity, Advent begins on the fourth Sunday before Christmas Day, or the Sunday which falls closest to November 30, and lasts through Christmas Eve, or December 24. When Christmas Eve falls on a Sunday, it is the last or fourth Sunday of Advent.
For Eastern Orthodox churches which use the Julian calendar, Advent begins earlier, on November 15, and lasts 40 days rather than four weeks. Advent is also known as the Nativity Fast in Orthodox Christianity.