The 12 days of Christmas begin on Christmas Day, which is December 25, and end just before the biblical time frame known as Epiphany. The twelfth night is January 5.
The song titled "The 12 Days of Christmas" made this period of time well-known. Mentions of gift-giving in the song tend to lead people to believe the period surrounds or leads up to Christmas Day. In reality, the period includes the days following the celebration of the birth of Christ and the amount of time it took the Magi, which are referred to as the Wise Men, to travel to the place of his birth.