Most of our traditions come from the early Romans. In the 3rd century, Emperor Aurelian established December 25th as the birthday of the "Invincible Sun." The Christian church in 273 AD selected that day as the birthday of Jesus. In 336 AD the celebration of the "Invincible Sun" became Christianized as the Birth of Jesus.
The tradition of decorating with evergreens comes from a combination of ancient cultures. Egyptians used green palm branches to symbolize life’s triumph over death on the shortest day of the year. The early Romans used evergreens to honor their god of agriculture during their winter festival of Saturnalia. The Druids decorated oak trees with golden apples. A custom of the Middle Ages was to decorate a tree with red apples to symbolize the feast of Adam and Eve on December 24th. Roman temples were decorated with evergreens to symbolize life’s continuity.
The stringing of lights or placing candles in windows also came from the ancient Solstice feasts. During the feast of Saturnalia, Romans kept lamps burning to ward off the spirits of darkness. In pagan Scandinavia, bonfires were made and the Yule log was burned to strengthen the Sun. Ancients lit candles, bonfires, and anything they could find to celebrate the return of the light."
http://www.astronomical.org/astbook/seasonal.html(nothing to do with Jesus orginally so celebrate the solstice)