Twelfth Night (also known as Epiphany Eve depending upon the tradition) is a Christian festival on the last night of the Twelve Days of Christmas, marking the end of Christmastide and the coming of the Epiphany. Different traditions mark the date of Twelfth Night as either 5 January or 6 January, depending on whether the counting begins on Christmas Day or 26 December. 6 January is celebrated as the feast of Epiphany, which begins the Epiphanytide season.

A superstition in English-speaking countries suggests it is unlucky to remove Christmas decorations before Twelfth Night (or to leave them up after Twelfth Night). Other popular customs include eating king cake, singing Christmas carols, chalking the door, having one's house blessed, merrymaking, and attending church services.

In many Western ecclesiastical traditions, such as the Lutheran and Anglican denominations of Christianity, Christmas Day is considered the "First Day of Christmas" and the Twelve Days are 25 December – 5 January, inclusive, making Twelfth Night on 5 January, which is Epiphany Eve. In some customs, the Twelve Days of Christmas are counted from sundown on the evening of 25 December until the morning of 6 January, meaning that the Twelfth Night falls on the evening of 5 January and the Twelfth Day falls on 6 January. However, in some church traditions only full days are counted, so that 5 January is counted as the Eleventh Day, 6 January as the Twelfth Day, and the evening of 6 January is counted as the Twelfth Night. In these traditions Twelfth Night is the same as Epiphany. However, some consider Twelfth Night to be the eve of the Twelfth Day (in the same way that Christmas Eve comes before Christmas), and thus consider Twelfth Night to be on 5 January. The difficulty may come from the use of the words eve, which is defined as "the day or evening before an event", however, especially in antiquated usage, it could mean simply "evening".

Bruce Forbes writes:

In 567 the Council of Tours proclaimed that the entire period between Christmas and Epiphany should be considered part of the celebration, creating what became known as the twelve days of Christmas, or what the English called Christmastide. On the last of the twelve days, called Twelfth Night, various cultures developed a wide range of additional special festivities. The variation extends even to the issue of how to count the days. If Christmas Day is the first of the twelve days, then Twelfth Night would be on January 5, the eve of Epiphany. If December 26, the day after Christmas, is the first day, then Twelfth Night falls on January 6, the evening of Epiphany itself.

The Church of England, Mother Church of the Anglican Communion, celebrates Twelfth Night on the 5th, and the term "refers to the night before Epiphany, the day when the nativity story tells us that the wise men visited the infant Jesus".

In AD 567, the Council of Tours "proclaimed the twelve days from Christmas to Epiphany as a sacred and festive season, and established the duty of Advent fasting in preparation for the feast." Christopher Hill, as well as William J. Federer, states that this was done to solve the "administrative problem for the Roman Empire as it tried to coordinate the solar Julian calendar with the lunar calendars of its provinces in the east."

In medieval and Tudor England, Candlemas traditionally marked the end of the Christmas season, although later, Twelfth Night came to signal the end of Christmastide, with a new but related season of Epiphanytide running until Candlemas. A popular Twelfth Night tradition was to have a bean and pea hidden inside a Twelfth-night cake; the "man who finds the bean in his slice of cake becomes King for the night while the lady who finds a pea in her slice of cake becomes Queen for the night." Following this selection, Twelfth Night parties would continue and would include the singing of Christmas carols, as well as feasting.