"Long before the
advent of Christianity, plants and trees that remained
green all year had a special meaning for people in the winter. Just
as people today decorate their homes during the festive
season with pine, spruce, and fir trees, ancient peoples hung
evergreen boughs over their doors and windows. In many countries it
was believed that evergreens would keep away witches,
ghosts, evil spirits, and illness... Early Romans marked the solstice
with a feast called the Saturnalia in honor of Saturn, the god of
agriculture. The Romans knew that the solstice meant that soon
farms and orchards would be green and fruitful. To mark the
occasion, they decorated their homes and temples with evergreen
boughs. In Northern Europe the mysterious Druids, the priests
of the ancient Celts, also decorated their temples with evergreen
boughs as a symbol of everlasting life. The fierce Vikings in
Scandinavia thought that evergreens were the special plant of the
sun god, Balder."
- Evergreen Traditions, History.com, (http://www.history.com/content/christmas/christmas-trees/evergreen-traditions)