Mistletoe Sacred to ancient druids and a symbol of eternal life the same way as Christmas tree. The Romans valued it as a symbol of peace and this lead eventually its acceptance among Christmas props. Kissing under mistletoe was a Roman custom, too. Anything goes nowadays. In old times they were simple, wood, paper, straw and often very intricate.
Themes follow the general taste of each time but national traditions can be discerned even now.
There are many roots of this custom. There is St. Nicholas the anonymous benefactor, there is the tradition of Magi giving precious gifts to Jesus, there is the Roman custom of giving gifts of good luck to children during Saturnalia. The Mediaeval Christmas music followed the Gregorian tradition. These songs continued to be religious and in Latin, though.