It was bad luck to put the tree up before Christmas Eve (it created family discord) or to leave it up after Twelfth Night (Nos Ystwyll), 6 January.
Mistletoe (Uchelwydd) has small green leaves and, in winter, white berries. The berries are poisonous! It was used in both winter and summer solstice celebrations by the Druids (Derwyddon) who would cut it from the oak, according to the Elder Pliny's Natural History, with a golden sickle. Mistletoe was usually banned from Christian churches due to Pagan association. It was believed that a tea of the leaves would relieve epileptics and wearing it would make a barren woman fertile.
It would be hung in the house on Christmas Eve and left there until the next year when a new sprig would replace it. Sometimes it would be decorated with ribbons. A kiss under the mistletoe meant a wedding soon.
The mistletoe was hung in houses to prevent lightning strikes, spells, and evil of all kinds. In Scandanavia, the plant is hung outside as an expression of peace and welcome.
Holly (celyn) was considered useful for treating fevers, dropsy and rheumatism, gout, and asthma. Native Americans used holly tea to treat measles. It was believed that to cure a child's tapeworm you had to get a child to yawn over a dish of holly and sage in water. This would draw the worm out through the child's mouth.
Holly berries were carried for luck. It was well-known that animals thrived if holly was hung up in the cowshed on Christmas Eve. It protected houses from bad luck and was acceptable in church. Holly leaves under the pillow on Christmas Eve would bring dreams of one's future mate. Thrashing chillblains (llosg eira, 'snow burns') until they bled was considered an instant cure. Female servants used to be chased by male servants and thrashed with holly until they bled. This was called 'holming'. It was also customary for farmers to bleed their animals at this time believing that the sun would bring new blood.
Holly picked on Christmas Day was a powerful curative agent. It was bad luck to stomp holly berries, to burn green holly, or to bring in holly flowers in during the summer.
Ivy (iddew or iorwg) was used in conjunction with the holly to bring fertility to the household. Ivy on the walls of a house, the inhabitants were protected from witchcraft. If the ivy withered, you could expect disaster. The house would be lost, either through no heir and infertility, or through financial emergency. Ivy leaves were good for divining one's future spouse. On Christmas Eve, go for a walk with ivy leaves in your pockets. The first member of the opposite sex to cross your path will be your future mate, even if they're currently married.
Medicinally, ivy leaves were used to wrap ulcers and abscesses. Wrapping a corn with a wrap of ivy leaves soaked in vinegar. Ivy vinegar was used during the Great Plague of London in 1665. It is said that ivy leaves worn in a wreath will prevent falling hair.
Rosemary (Rosmari) was considered to be holy and magical. According to tradition, the Rosemary grows for 33 years until it reaches the height of Christ at his crucifixion, then only grows outward after that.
Rosemary is the plant of rememberance. A sprig of rosemary reminds those present of those who were with them last year but are no longer present.
The Yew (ywen) and the Cypress (cypreswydden) were considered spring or Easter plants.
Some Christmas beliefs:
If the sun shines through the apple trees on Christmas Day, there will be a good crop the coming season.
Bees hum the 100th Psalm at midnight in their hives.
Farm animals speak human tongue at midnight, but trying to hear them will result in an early death.
Cattle of seven years old turn east and bow at midnight.
All dogs who howl on Christmas Eve must be shot or they will be mad by New Year's Eve.
Bread baked on Christmas Eve will cure diarrhea and stomach disorders.
Sit under a pine tree on Christmas Eve and you will hear angels sing, followed by your demise.
Never make a broom during the twelve days of Christmas.
For Irish traditions, go to Conrad Bladey's Irish Pages/ Seasonal Celebrations