From, "Another Look at Christmas in the Eighteenth Century," By David DeSimone in The Colonial Williamsburg Interpreter, vol. 16, no. 4, winter 1995-96.
"How long was the Christmas season?
The holiday, or more accurately the holy days of Christmas/Epiphany, fall into a forty- day cycle. This cycle was (and still is) a commemoration of the infancy narratives found in the Gospels of Saint Luke and Saint Matthew. Four major events involving Jesus, Mary, and Joseph are commemorated in the forty-day cycle. They are:
December 25: The Nativity of Jesus
January 1: The Circumcision of Jesus (Eight Days after Christmas)
January 6: The Epiphany of Jesus (Twelve Days after Christmas)
February 2: The Purification of the Virgin (Forty Days after Christmas)*
Some of the most sacred holy days are observed within the octave of Christmas. The octave week (an eight-day observance) began on Christmas Day and included December 26-- Saint Stephens Day; December 27--Saint John the Evangelists Day; and December 28--Holy Innocents Day. The octave week festival ended with the Feast of the Circumcision on January 1. It cannot be emphasized enough that amid the joy of the season, the holy days of Saint Stephen and the Holy Innocents were a solemn reminder of the darker side of humanity. In fact, Saint Stephen and the Holy Innocents were martyred because of Christs coming.
. . .
While the observance of these sacred days was over, it did not signal the end of the liturgical season of Christmas. Eight days after Christmas, January 1, was the celebration of the Circumcision of Christ. Twelve days after Christmas was the Feast of the Epiphany, or the manifestation of Christ to the Gentiles. Finally, forty days after Christmas was the Feast of the Purification of the Virgin Mary."
*Also know as Candlemass and celebrated in a popular sense in the United States as Groundhog Day.