When I was younger, I believed the Twelve Days of Christmas, as referred to in the song of the same name, occurred before the holiday—thus from December 13^th through the 25^th. Later, I learned that the twelve days begin on Christmas and extend until Twelfth Night, January 5^th. Twelfth Night, also called Epiphany Eve, is the traditional last day of the Christmas holiday festivities. It also marks the adoration of the Magi, and many cultures celebrate it as almost a second Christmas Eve. 

Beginning in Tudor England, Twelfth Night was often commemorated with a large, festive party to mark the end of the Winter Festival. As tradition goes, for the evening of Twelfth Night the world would be turned upside down. The King and nobility would become peasants and vice versa. A Twelfth Night Cake was baked to contain one bean, and the man to find it became the Lord of Misrule. The Lord of Misrule directed the feast, and could order others to do silly or humorous things at his whim.  It was all part of the merry-making of the eve. Of course, it also behooved the Lord of Misrule to remember that after midnight he was no longer in charge, or else he might regret angering the King or lords. Often times, a pea was also baked into the cake, and the woman to find it became the Queen of the feast. Shakespeare penned his play Twelfth Night specifically to be performed at Twelfth Night celebrations. 

William Harrison Ainsworth's Mervyn Clithroe's Twelfth Night Party by "Phiz" (c 1840)

Even today there are numerous foods and traditions still associated with Twelfth Night. In parts of England and France the Twelfth Cake is still baked, as well as tortell in Catalonia, Spain, and king cake in New Orleans. Wassail is customarily served as the beverage of choice. Twelfth Night also marks the start of the Carnival season, which ends with Mardi Gras Day. In the eastern Alps, a custom called Berchtenlaufen is still observed where several hundred young, masked men run through the streets driving out evil spirits with whips and bells. It is also believed to be unlucky to leave your Christmas decorations up after Twelfth Night, giving us some extra motivation to take the tree down in a timely manner.