When you meet your girlfriend/boyfriend under some mistletoe you are supposed to kiss her/him, this is a well known symbol of peace and luck.
This tradition has it s origins in the North and Celtic cultures, on which Balder Balder, the son of the Norse goddess Frigga, was killed by an arrow made of mistletoe by Loki, and evil spirit, she wept tears of white berries which brought him back to life. Overjoyed, Frigga blessed the plant and bestowed to kiss all who passed beneath it. When Catholicism became the religion of these cultures some of their ancient symbolism were conserved as it happened with the mistletoe.
The holly was traditionally used in the ancient Roman Empire in the Saturnalia Fest celebrated in honour to Saturn, during this times roman houses were decorated with holly and so used do the Christians, afraid of the persecution hey suffered. When most Romans became catholic the holly kept being used as decoration and this tradition continued to those days.