Twelfth Night is a Christian holiday celebrated after the Twelve Days of Christmas and marking Epiphany. Many SCA chapters have Twelfth Night events, since it is conveniently outside of the central Christmas holiday, allowing for greater attendance than an actual Christmas event would.
Twelfth Night was celebrated in various ways in period, depending on time and culture. The tradition of a Lord of Misrule seems to have been fairly common.
Shakespeare, quite famously, has a play entitled 'Twelfth Night' which was written for Twelfth Night festivities.