Christmas is a Christian holiday which celebrates the birth of Jesus Christ. Christmas is typically celebrated on December 25th. The word "Christmas" comes from "Christ's mass," and the Roman Catholic church and most Protestant denominations observe Christmas after sundown on the 24th of December. Some Eastern Orthodox Church(s) practice it on January 6th, combining the day with Epiphany. Orthodox practice a Christmas Fast lasting 40 days (November 15 to December 24).
The story of the birth of Christ is told in the opening chapters of the Gospel of Luke and the Gospel of Matthew. Each contains different elements of the Christmas story. The visit of the Archangel to the shepherds and the birth of Jesus in a manger are from Luke. The story of the Star of Bethlehem and the visit from the Wise Men bringing gifts of "gold, and frankincense and myrrh" are from Matthew.
It is worthwhile to note that December 25th is likely not the date of Jesus' actual birth. In the Biblical description of Jesus' birth, He was visited by three shepherds who had been out that night with their sheep. Shepherds would not have been out with their sheep in the winter. Jesus' birth date is more commonly agreed to be some time in the autumn. It was not until A.D. 440 that the church officially proclaimed December 25 as the birth of Christ. This was not based on any religious evidence but on a pagan feast. Saturnalia was a tradition inherited by the Roman pagans from an earlier Babylonian priesthood. December 25 was used as a celebration of the birthday of the sun god. It was observed near the winter solstice. 
In Western countries, during the Christmas season people generally hang up colored lights, place a Christmas tree (typically an evergreen tree) in their house, sing carols (songs), and exchange gifts. Gift-giving commemorates the gifts given to the Christ child by the Wise Men and is symbolic of the fact that Jesus came as gift to mankind from God.
Festivals have always been celebrated around the time of the solstice. Two thousand years before Christ, the early Mesopotamians celebrated a 12 day new year around that time, which is still reflected in our custom of the 12 Days of Christmas. Their chief god, Marduk, was supposed to battle annually against the forces of chaos, which the Mesopotamians believed were making the sunlight dwindle. The Mesopotamian King was supposed to be killed to go fight at Marduk's side. However, the king usually substituted a condemned prisoner and let him spend a day as the king. He was treated like the king for a day, then at the end of the day he was stripped and killed in the king's stead.
Some scholars assume that the historical Jesus was born in the Spring, rather than how Christian tradition indicates. It wasn't until around AD 350 that 25 December was appointed to be the Mass of Christ, probably to coincide with the date of the Winter Solstice. The early Catholic Church concurred, probably in order to draw parishioners away from licentious pagan festivals occurring at about the same time of year. The Romans decked their halls with green laurels and trees decorated with candles. This feast, called Saturnalia, was a time for masquerades and feasts and the trading of presents, including "lucky fruits," the ancestor of today's fruitcake.
Christmas is a relatively unimportant holiday in the Christian liturgy (less important, for example, than the Feast of the Epiphany). Early Protestants, including the Puritans who settled in Plymouth, did not celebrate Christmas, which they denounced as Papist.
Many of the customs we have come to associate with Christmas developed in Victorian England, notably the Christmas Tree (which originated in Germany and was introduced in England by Prince Albert). The concept of Christmas as a family-centered celebration and a time of generosity, good will, and friendliness to neighbors, was promoted and codified by Charles Dickens in A Christmas Carol.
In the United States, the image of Santa Claus (i.e. Saint Nicholas) and his reindeer has been shaped by Clement Moore's poem, "A Visit from Saint Nicholas" (with the famous opening line "'Twas the night before Christmas.") In other countries, the depiction of Santa vary widely. For instance, in The Netherlands, Santa Claus is shown in a Catholic bishop's miter. In some countries, Saint Nicholas is not associated with the holiday at all; for example, in Spain it is the Three Wise Men who bring gifts to children, and in Hawaii it is the jolly native Kamaunamauna. However, the tradition of Christmas gift-giving is prevalent in all cultures.
In the United States, during the twentieth century, gift-giving assumed a greater and greater role, and by mid-century had become of great commercial importance, to the point where some felt the religious aspects were becoming forgotten. As Stan Freberg put it in 1958, "There are two S's in Christmas and they're both dollar signs." The single week before Christmas currently accounts for 25% to 30% of all retail sales.
Modern-day cultural significance
The tension between religious and secularized Christmas has waxed and waned periodically over the years. The pendulum swung toward secularization in the middle of the twentieth century. One marker was the emergence of songs like I'll Be Home For Christmas (1943), The Christmas Song, (1946), and Silver Bells (1951), which have become a beloved part of Christmas in America despite their lack of any religious content. Bing Crosby, a devout Catholic, initially was reluctant to sing the secular 1942 song White Christmas, which was to become his biggest hit.
In recent years, the celebration of Christmas has become part of the "culture wars." Fox News analysts John Gibson and Bill O'Reilly claim that Christmas has come under attack by liberals. Some Christian conservatives want an increased recognition of the religious core of the holiday, and want to maintain a separation between Christmas and other holidays occurring at the same time of year.
Outside of America, in Britain for example, powerful lobbies are trying to remove all the Christian connotations in the name of being politically correct. This is similar to the idea of CE in that it ignores the religious/historical basis of Christmas.
Christmas trees may be mentioned in the book of Jeremiah as pagan idols, in verses strikingly similar to the popular Christmas song, "Deck the Halls".
Thus saith the LORD, Learn not the way of the heathen, and be not dismayed at the signs of heaven; for the heathen are dismayed at them. For the customs of the people are vain: for one cutteth a tree out of the forest, the work of the hands of the workman, with the axe. They deck it with silver and with gold; they fasten it with nails and with hammers, that it move not. They are upright as the palm tree, but speak not: they must needs be borne, because they cannot go. Be not afraid of them; for they cannot do evil, neither also is it in them to do good. Jeremiah 10:2-5 (KJV)
- Carols by Candlelight
- Christmas Carol
- Santa Maria
- Saint Nicholas
- Christmas Paintings
- Christmas in the Holy Land: an essay
- Christmas in Conservapedia
- I Believe in Father Christmas