"Christmas, according to the liturgical calendar, isn’t just a day, but a season. Many Christians know the song “The Twelve Days of Christmas” or recall that Shakespeare’s Twelfth Night is about the last night of Christmas. But what defines the Christmas season?
The 12 days of Christmas are a reversal of the season of Advent. Traditionally, Christians have fasted during Advent as we await the celebration of the birth of our Savior. The period from Christmas Day to the day before Epiphany (the celebration of the magi visiting Jesus, the baptism of Jesus and the beginning of his earthly ministry) is a period of daily feasting. This is surprising, given the reason for celebrating several of the days."
This article in Living Lutheran magazine gives the full run-down:
"What are the 12 Days of Christmas"