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.
Today people still feel uneasy about leaving the Christmas decorations up after Twelfth Night.
Despite decorations now being made of foil or paper, and even though the tree-spirits are long forgotten, the superstition still survives.
