Once a great celebration, now mainly remembered as the date when you're supposed to take your decorations down.
But what is it? Here's the basics on Twelfth Night:
When is it?
Twelfth Night falls on January 5 - the night before the Twelfth Day of Christmas (Epiphany, when the Three Wise Men were said to arrive at the stable where Jesus was born).
Why is Twelfth Night not on January 6?
To our ancestors, the end of the day was when the sun came down - so the night was actually the beginning of the next day.
So for them, Christmas Day started at sunset on December 24 (Christmas Eve) and would continue until nightfall on the 25th, which began the Feast of Stephen (Boxing Day, as we now call it, which was the actual First Day of Christmas).
If you follow it along, the Twelfth Night of Christmas was the night before January 6.
So when should I take my decorations down?
Since Victorian times, the tradition has been to take decorations down on Twelfth Night. So technically, the answer is January 5.
But most people now think of January 6 as being Twelfth Night. So unless you're majorly superstitious, you can probably leave them until then.
It's also worth remembering that until Victorian times, our ancestors left their decorations up until Candlemas on February 2 (having not put them up until Christmas Eve).
Read more: When should you take down your decorations?
How is Twelfth Night celebrated?
In times past, it was a big celebration, and a time to play practical jokes on your friends, family and neighbours - including hiding live birds in pies, as in the rhyme Sing a Song of Sixpence.
In Britain, this was a rich, heavy fruitcake, which traditionally contained a bean. If you got it, you were King or Queen of the Bean and everybody had to obey your orders, often to do ridiculous things.
It was also a time to eat spicy, gingery foods.
Wassail is a kind of hot mulled cider, traditionally drunk around Christmas.
Wassailing refers to singing and drinking the health of trees on Twelfth Night, to awaken apple trees and to scare away evil spirits, ensuring a good harvest in Autumn.
Traditional Christmas plays, a kind of early pantomime, usually featuring the story of George and the Dragon.
Shakespeare's Twelfth Night
The play is believed to have been written around 1601/02 as entertainment for close to the Christmas season, and its plot of disguises and confusions seems to fit the spirit of the occasion.
This was traditionally lit at the beginning of Christmas, and burned until the Twelfth Night, when its remains were kept to kindle next year's fire - and for good luck.
Is Twelfth Night still celebrated today?
Many places throughout the UK still mark the occasion, and celebrations take place in London's Bankside each year - though often not falling on Twelfth Night itself, but rather the weekend closest to the date.