Historically, Christmas commemorates the birth of Jesus of Nazareth to a young maiden from Galilee. Theologically, Christmas is the celebration of the incarnation of God in Jesus the Christ, the self-revelation of God to the world in human form for the reconciliation of humanity to Himself. All the details of the various accounts concerning Jesus’ birth revolve around that central truth (see The Meaning of Christmas below).
While we most often think about Christmas as a single day, it is actually a season of the year. In its popular sense, it extends four weeks before Christmas Day and for two weeks after. However, the time before Christmas is a special season called Advent, comprising the four Sundays before Christmas Day. While the entire season of Advent, Christmas, and Epiphany can be seen together, they each have distinctly different roles in the Church year. The term "Advent" means "coming" and is a season of expectation and hope, the time of waiting for the coming of the Messiah that is celebrated at Christmas. This time of waiting symbolizes the waiting throughout the Old Testament for the new act of God that would bring deliverance to his people. For Christians this season of expectation also symbolizes the waiting in anticipation for the Second Coming of the Christ when he will return and restore all things (see The Season of Advent).
Contrary to advertising campaigns that tout Christmas as beginning with Advent (or Halloween!), the actual Christmas Season in most Western church traditions begins at sunset on Christmas Eve, December 24, and lasts through January 5. Since this time includes 12 days, the season of Christmas is known in many places as the Twelve Days of Christmas. January 6 is usually celebrated as Epiphany, although it carries different significance in various church traditions. Due to different calendars in use in various eras and locations of the church, some cultures and church traditions celebrate Christmas on January 6 (in the older Julian calendar still used as the religious calendar in Eastern Churches, January 6 corresponds to December 24 on our modern calendar).