Lighted trees, midnight Masses, advertising blitzes — we know Christmas when we see it, hear it, smell it and feel it.

And in the glowing goodwill of the season, it's easy to believe that's the way it has always been.

In reality, Christmas has changed dramatically over the last two millennia to become much more than the sum of its parts.

Today Christmas is as important to the economy as it is to spirituality.

The truth is our contemporary Christmas is a holy hodgepodge of sacred traditions, literature, charity, history, art, chronological calculation and marketing magic.

So as people put up trees, hit the stores, deck the halls and fold their hands, it might be worth pausing for a moment to consider how we got here.

Let's start with Dec. 25.On this day an estimated 2.1 billion Christians celebrate the birth of Jesus of Nazareth, whom they believe to be the son of God.It's as good a day as any to celebrate, but scholars agree that Dec. 25 is almost certainly not Jesus' birthday. A better guess is sometime in February during the lambing season when shepherds would have watched their sheep at night.So how did Dec. 25 become the holy day? There are many theories. Here's the most common.For about 300 years the earliest Christians did not celebrate Christmas. Because pagans often celebrated the miraculous births of their gods, early Christian thinkers opposed the celebration of birthdays on principle.But by the fourth century they overcame their reluctance and became willing to honor Jesus on his birthday. There was only one problem.Nobody knew Jesus' birthday.The Bible gives neither day nor date.Most researchers believe Christian leaders set the day as Dec. 25 to compete with pagan rituals celebrated around the same time.One of the largest Roman festivals, Saturnalia — which honored Saturns, the god of seed and sowing — took place around the winter solstice, or the shortest day of the year. On the calendar used by ancient Romans, the winter solstice fell on Dec. 25.That was also the date sun-god worshippers (devotees of Sol and Mithra) celebrated the Birth of the Invincible Sun. During the fourth century, Christianity was still trying to fully establish itself. By putting Christmas on Dec. 25, leaders tried to supplant the celebration of the birth of the sun with a celebration of the birth of the Son.Sometime in the middle of the fourth century, researchers believe, either Pope Julius or Pope Liberius officially adopted Dec. 25 as the Feast of the Nativity. During the next several centuries the feast spread throughout other Christian communities, taking various forms.

The legend of Santa Claus traces its roots to St. Nicholas, a minor saint from the fourth century.He was said to have carried a sack of toys for children and to have performed many kind and generous deeds in secret, expecting nothing in return. Historians disagree on the truth of such stories.True or not, they inspired the creation of Santa Claus, whose magical and generous nature spread throughout the arts and popular culture.Of course, Santa was not always jolly or plump. In colonial days he was often thin and beardless. Santa's modern image formed thanks to imaginings by Washington Irving and "The Night Before Christmas" by Clement C. Moore.It grew stronger thanks to numerous Christmas songs, artwork by cartoonist Thomas Nast (which depicted Santa as a jolly plump man with white beard in a red suit) and through iconic Coca-Cola ads.

Christmas wasn't made an official holiday in the U.S. until 1836, and then only in Alabama. It didn't become a federal holiday until 1870.One of the most powerful influences in changing the attitude of those still resistant to the full celebration of Christmas was Charles Dickens' "A Christmas Carol," published in 1843. Many historians say the transcendent work may have been the single most important factor in popularizing attitudes toward what would become our contemporary Christmas.The work succeeded in effectively demonizing anyone who would dare oppose such a season of joy and giving.

During the 20th century, many advertising campaigns helped to promote fun and fantasy at Christmastime.Rudolph the Red-Nosed Reindeer, for example, sprang from the imagination of 34-year-old Montgomery Ward copywriter Robert May.In 1939 the Chicago-based store asked May to come up with a Christmas story they could give away to shoppers as a promotional gimmick. Wards gave away more than 2 million copies the first year and countless more millions over the next several years. Rudolph later inspired popular songs and cartoons and found a permanent home in mainstream Christmas lore.There was no stopping Christmas from there. It gained an unshakable foothold through TV, movies, marketing, advertising, plays, parades, pageants and parties.Some say we have gone overboard and ruined the true meaning of the holiday with rapacious spending.But others note that, if not abused, commercialism is not all bad. If, as some charge, commercialization has taken Christ out of Christmas, it also has put much of the magic and fun into it, making the holiday the unrivaled event that it is today.