It's getting to the time of year when a certain segment of the religious right -- the folks who think simple faith is preferable to studying theology and their own church history -- start whining about "The War on Christmas." You will hear them complain about Neopagans and Secular Humanists co-opting Christmas for their own holiday celebrations. Some of them will make statements about Christmas trees being inappropriate for anyone but Christians (or with any other name but Christmas tree).
There is no language in the gospels indicating that Jesus of Nazareth was born in December. In fact, the only seasonal clue -- the shepherds keeping watch over their flocks at night -- would seem to indicate that the nativity took place in the Spring, during lambing season (the only time of year in which shepherds spend the nights in the fields with their flocks).
Most cultures have some sort of celebration that is tied to the Winter Solstice. The minor Jewish festival of Channukkah is overtly called "the festival of lights" and its theme is light lasting longer than expected. The Jews' Roman occupiers celebrated Saturnalia, a multi-day festival that included family gatherings, feasting and the giving of gifts.
In addition to the official Roman state religion, many Roman soldiers practiced Mithraism, a mystery religion (meaning its doctrines were kept secret from all but the highest-level initiates) that originated in Persia. Our knowledge of Mithraism is sketchy (since it was a mystery religion that completely died out in ancient times), but we know it involved a god named Mithras who was born on the Winter Solstice and later sacrificed a celestial bull to save mankind from the darkness.
When Christianity became an official State religion of the Roman Empire, it began to compete openly with the existing pagan religions for followers. Forcing people to give up their cherished holiday traditions is no way to win converts. Thus, the church decided that Jesus was born on December 25 (the Bible didn't directly say otherwise, after all), and declared that the faithful could celebrate the Nativity with a Christ Mass and feast. The Saturnalia traditions were largely absorbed into the 12-day Christmas celebrations (culminating with Epiphany, which commemorates the visit of the Magi with their fabulous gifts).
Northern European pagans had their own Solstice celebrations. The Scandinavian and German peoples burned a Yule log to represent the returning sunlight and brought evergreen trees (tannenbaums) inside, because they defy winter by remaining lifelike rather than dropping their leaves. When Christian missionaries proselytized among the northern tribes, they let them keep their old traditions, attaching them to the new Christmas celebrations.
After about 1500 years, Christmas traditions are indeed venerable. However, it is ignorant to assume that Christians have a monopoly on December celebrations, or that non-Christians have no right to decorate evergreen trees and exchange gifts.
In December, I will wish my family and Christian friends a Merry Christmas (or Feliz Navidad or Mele Kamikimaka, depending on my mood and how much I am longing for the tropics). I will wish my Jewish friends and relatives a Happy Channukah. I will wish my pagan friends and relatives a Good Yule. And to everyone, I will wish Happy Holidays (because no matter which of the above holidays we celebrate, most of us celebrate New Year's Eve as well).