In the Northern hemisphere, many ancient people believed that the sun was a god & that winter came every year because the sun god had grown weak. They celebrated the solstice, because it meant that at last the sun god would begin to get well. Evergreen boughs reminded them of all the green plants that would grow again when the sun god was strong & summer would return.
The ancient Egyptians worshipped a god called Ra, who had the head of a hawk & wore the sun as a blazing disk in his crown. At the solstice, when Ra began to recover from his weakness, the Egyptians filled their homes with green palm rushes symbolizing the triumph of life over death.
Early Romans marked the solstice with a feast called the Saturnalia in honor of Saturn, the god of agriculture. To mark the occasion, they decorated their homes & temples with evergreen boughs.
In Northern Europe the 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.
Decorating one’s house with natural boughs has been a Christmas tradition since Celtic times in England. Boughs of holly with their bright red berries were especially coveted.
"Heigh ho! sing heigh ho! unto the green holly:
Most friendship is feigning, most loving mere folly:
Then, heigh ho, the holly!
This life is most jolly.”
- William Shakespeare, As You Like It