The answer to this question is important for traditionalists who like to take down their Christmas decorations before Twelfth Night passes (risking bad luck, for the superstitious).
Twelfth Night is the Eve of Epiphany and if you count the first day of Christmas at the 25th December, then Twelfth Night becomes the night of January 5th.
For Christians, Epiphany celebrates the arrival of the Magi (Three Kings or Wise Men) to visit the Christ Child and it is the revelation of Jesus Christ to the world.
Twelfth Night is also one of Shakespeare’s most performed plays. A romantic comedy, Twelfth Night is about love, suffering, gender identity and the folly of ambition. As relevant today as it was in the early seventeenth century.
Why Twelfth Night? Apparently, at one time, the feast of Twelfth Night was when social hierarchy became rather muddled and social strictures loosened during the feasting period. A bit like the proverbial office Christmas party!