Twelfth Night is a Christian festival that falls on January 5th, the last day of the 12 days of Christmas. In our Anglican liturgical calendar, Twelfth Night is also Epiphany Eve. Many of the customs surrounding Twelfth Night focus on the coming of the Three Kings and the transition from Christmastide to the Epiphany season.
Popular Twelfth Night customs include eating king cake, singing Christmas carols, chalking the door, having one’s house blessed, merry-making, and attending church services (external links).
[Adapted from “Twelfth Night (holiday)” on Wikipedia.]