Under commercial terms, it starts the first time "Last Christmas" is heard on the radio in early November, and it ends when the after Christmas sales begin around December 27th. Under religious terms, the Christmas season runs from Christmas Eve through the feast of the Baptism of the Lord, which is the Sunday after the Epiphany. The Epiphany is when the kings come to see baby Jesus.
The Octave of Christmas concludes on January 1st, which is a holy day of commitment. From December 25th up to February 2nd, the Vatican leaves up their tree and nativity until then. This timeframe is, altogether, forty days. Traditionally speaking, take out the strict and traditional religious variables, and Christmas is for one day, which is December 25th.