Twelfth Night is the night before Twelfth Day.
Twelfth Night (5th January) is when all Christmas Decorations should be removed so as not to bring bad luck upon the home. If decorations are not removed on Twelfth Night, they should stay up all year.
Why is it bad luck to leave the decorations up after Twelfth Night?
Long ago it was thought that leaving the decorations up would cause a disaster. People believed that tree-spirits lived in the greenery (holy, ivy etc) they decorated their houses with. The greenery was brought into the house to provide a safe haven for the tree-spirits during the harsh midwinter days. Once this period was over it was necessary to return the greenery back outside to release the tree-spirits into the countryside once again. Failure to do this would mean that vegetation would not be able to start growing again (spring would not return), leading to an agricultural disaster.
It was also thought that, if you left the greenery in the house, the tree-spirits would cause mischief in the house until they were released.