Santa Claus himself dates back to a man named Nicholas, later to become Saint Nicholas, supposedly able to do more than the good deeds he was known for, even bringing the dead back to life. Strangely, the area where he lived and did his good deeds is in modern day Turkey. Our modern vision of Santa Claus is given to us by the Coca-Cola Company, who popularized the image of a fat man in a red suit in their advertising.
Stockings hung by the chimney come directly from Saint Nicholas. Legend has it that a nobleman was saddened by his wife's death and foolishly wasted his fortune, leaving him without a dowry for his three unmarried daughters. Nicholas secretly dropped coins down their chimney at night, and the coins fell into the stockings the girls had hung at the chimney to dry, giving us the tradition of stockings being filled with goodies and the legend of Santa coming down the flue.
Christmas trees themselves date back perhaps thousands of years. At that time in northern Europe winters were difficult, even deadly, and the evergreen remaining vital through the cold became a symbol of life sustaining through the winter, and one was often dragged into the home. Later in Germany it became a spruce, and legend has it that Martin Luther admired stars glowing through the trees, so he placed candles on a spruce to emulate the beauty of the night.
The first modern tree was created by Thomas Edison employee Edward Johnson in 1882, strung with 80 red, white, and blue bulbs, all turned upward as they thought that prevented the bulbs from burning out as quickly.
Ornaments are simply modern variants of using apples to decorate the tree, symbolic of the Garden of Eden.
Holly also was inspired by the Bible, used as garland on the tree or in wreaths, it reminded the people of the crown of thorns and Jesus' suffering.
America from the time of the Puritan settlers until the early 1800's did not celebrate Christmas, as it had been a rowdy holiday of debauchery in England, and for decades after the American Revolution all things British were not thought highly of. Only after decades did American long for a winter holiday, and powerful figures in American history used the writings of Dickens and others to popularize a more civil, family oriented holiday than the Mardi Gras like British Christmas of old.
A Christmas dinner is no more than a winter feast in disguise. In more primitive times it was simple. You could sustain a few breeding cattle through the winter, the rest had to be killed and eaten before they spoiled, so what better time to avoid the cold, stay inside, feast with family and friends, and celebrate Christmas.
Poinsettias name comes from U.S. ambassador to Mexico Joel Poinsett. He saw the flowering plant in 1828 and thought it would look great in America at Christmas. Everyone agreed.
Mistletoe was thought to have magical powers as it grew in trees, stayed green all winter, and had no roots. The plant was associated in Scandinavia with the goddess of love, Frigga, and this is probably where the idea of sneaking a kiss under the mistletoe came from. Not a bad idea.