Though Christmas is a Christian celebration, many of its traditions are adopted from ancient Celtic observances of Winter Solstice.
Consider the Christmas tree. It plays a major role in Christmas festivities. But the tradition was likely adapted from the Celts, who saw evergreens as important in warding off evil spirits. They would decorate them with sun symbols and gifts to the Celtic gods.
The story behind mistletoe is even more interesting. Sneaking a kiss beneath it has been a Christmas tradition for centuries. But its first connections to love are said to have originated in Norse mythology, when Frigg, the Goddess of Love was so grateful to have her son, Baldaur brought back to life after being mortally wounded with a mistletoe arrow that she declared mistletoe the symbol of love and vowed to kiss all those who passed under it. (I wonder if the Norsemen in The Druid and the Dragon brought mistletoe with them when they invaded King Redmond's kingdom.) At any rate, the ancient Celts also revered mistletoe. Because the plant was able to survive and bloom even in the most bitter winter weather, they saw it as a symbol of vivacity, and it was given to people and animals as a fertility enhancement. (Knowing that, you might want to think twice about being caught beneath a sprig in the future!)
Who doesn't love a good yuletide fire! It is a cozy place to sing carols and drink eggnog. The Celts believed that during the darkest days of winter, the sun stood still for twelve days, So they burned yule logs to counter the long winter nights and bring good luck.
The Druids celebrated the Winter Solstice with the festival of Alban Arthan, also known as Yule. These were the shortest days of the year, and it was believed a great ritual was needed to turn things around and encourage the sun to stay longer each day. So great stone circles and burial mounds such as Newgrange were constructed and positioned in such a way as to focus the sunlight like a laser, an act the Druids believed would alter the sun's journey and start the move to spring.
And now you know.