Holly was traditionally used to decorate houses in mid winter as a sign of goodwill and to endow all those who entered with long and healthy lives, later because of Christian associations it became increasingly used as a Christmas decoration.
Holly was traditionally a man’s plant (bringing strength and protection, with ivy believed to do the same for women), but it was said whoever brought the holly into the house for Christmas would be in charge for the rest of the year!
Holly has many Christian associations, one of which is that Holly trees grew leaves out of season in order to hide Jesus and his family when they were fleeing into Egypt, another being that the cross was made of holly.
Like many of the Christian traditions a more likely reason for decorating with holly were the pagan associations of this plant as one of the sacred evergreens, one of the few things still green at midwinter and therefore especially favoured by the sun. It was easier to incorporate this into the new religion than ban it, especially as it was already a common Roman practice to give sprigs of holly to friends at Saturnalia (the Roman mid winter feast) to ward off evil spirits.