Call it the paradox of Christmas: every winter, just as the seasonal darkness is at its bleakest, Christians around the world celebrate the year's most jubilant holiday.

Yuletide cheer is a welcome break from the Northern Hemisphere's winter doldrums - and a deliberate one, says a historian who has charted the origins of the Christmas holiday.J. Patout Burns, professor of Christian thought at Washington University in St. Louis, has spent most of his life studying the origins of Christmas and other religious traditions.

Burns says it is no coincidence that Christmas occurs just after the Dec. 21 winter solstice. According to his research, when Christians first began observing the birth of Christ somewhere around the third century A.D., they purposely chose a solstice celebration.

"There is no basis in the New Testament for the belief that Jesus was born in late December," Burns said. "The early Christians placed the birth at the time of the winter solstice for symbolic reasons.

"Jesus, to the Christians, was the light of the world - he saved humanity from darkness. So, back then, if you asked what would be the most appropriate time for him to be born, the obvious answer was: at that point in the year when light begins to increase and to overtake darkness," he said.

Burns's theories about the pragmatic nature of Christmas traditions have raised more than a few eyebrows over the years.

In his view, the celebration of Christmas has evolved greatly over the past 16 centuries. He believes the evolution has been governed not by religious considerations but by what he calls "social engineering."

Early Christians lived at a time when nature and the change of seasons were dominant forces in religious beliefs. Hence, Burns says, Christmas coincided with the winter solstice and Easter occurred around the time of the spring equinox.

Around the 12th century, medieval Christians took a more humanistic view of God and theology. "We see a more human emphasis on the infant Jesus," said Burns. "Our modern use of the Nativity creche dates from around that time."

The evolution of Christmas continued, Burns noted, and, thanks to social engineering, the contemporary holiday has become less of a religious observance and more of a public, secular celebration.

"To some extent, what you're seeing today is a great ambiguity," he said. "Look at the legal battles that take place every year when people demand that creches be removed from public places.

"Christmas is, after all, a commercial success. There is more pressure now to maintain it as a public feast, even if some people wish the hullabaloo would go away and be replaced by private, religious observance," he added.

Still, some traditions die hard. After the first celebration of Christmas 1,600 years ago, the holiday continues to brighten spirits at the same time that increasing daylight brightens the world.

Of course, that could eventually change, Burns said. In terms of seasonal light, "the most important days of the year for us now are the days when we change our clocks by an hour and go from daylight-saving to standard time or vice versa," he said.

Might some future Christmas be celebrated on a Sunday in late October? Burns laughed. "It's an interesting idea," he said. "A very interesting idea."