Twelfth night is now celebrated on 5th January. This was originally the last day of the 12 Days of Christmas celebrations and was commemorated by William Shakespeare in his play Twelfth Night. In ancient times this was the night for the Lord of Misrule to dominate the festivities, leading the games and activities, in more recent times it become traditional to take down all the Christmas decorations on this day.
Some places still celebrate with Wassailing where the spirits are scared out of the apple trees to enable a good crop later in the year. This is mainly continued in the cider producing areas of Southern Britain today and tends to be celebrated on 17th January (which was the original date for Twelfth Night before calendar changes).