Scholars believe that during the 4th and 5th centuries in Spain and Gaul, Advent was a season of preparation for the baptism of new Christians at the January feast of Epiphany, the celebration of God’s incarnation represented by the visit of the Magi to the baby Jesus (Matthew 2:1), his baptism in the Jordan River by John the Baptist (John 1:29), and his first miracle at Cana (John 2:1). During this season of preparation, Christians would spend 40 days in penance, prayer, and fasting to prepare for this celebration; originally, there was little connection between Advent and Christmas.
By the 6th century, however, Roman Christians had tied Advent to the coming of Christ. But the “coming” they had in mind was not Christ’s first coming in the manger in Bethlehem, but his second coming in the clouds as the judge of the world. It was not until the Middle Ages that the Advent season was explicitly linked to Christ’s first coming at Christmas.
Today, the season of Advent lasts for four Sundays leading up to Christmas. At that time, the new Christian year begins with the twelve-day celebration of Christmastide, which lasts from Christmas Eve until Epiphany on January 6. (Advent begins on Sunday that falls between November 27th and December 3rd each year.) Advent 2018 begins on Sunday, December 2 and ends on Monday, December 24.