More than a Hallmark holiday, Valentine's Day, like Halloween, is rooted in pagan partying.

The lovers' holiday traces its roots to raucous annual Roman festivals where men stripped naked, grabbed goat- or dog-skin whips, and spanked young maidens in hopes of increasing their fertility, said classics professor Noel Lenski of the University of Colorado at Boulder.

The annual pagan celebration, called Lupercalia, was held every year on February 15 and remained wildly popular well into the fifth century A.D.—at least 150 years after Constantine made Christianity the official religion of the Roman Empire.

"It is clearly a very popular thing, even in an environment where the Christians are trying to close it down," Lenski said. "So there's reason to think that the Christians might instead have said, OK, we'll just call this a Christian festival."

The church pegged the festival to the legend of St. Valentine.

According to the story, in the third century A.D. Roman Emperor Claudius II, seeking to bolster his army, forbade young men to marry. Valentine, it is said, flouted the ban, performing marriages in secret.

For his defiance, Valentine was executed in A.D. 270—on February 14, the story goes.

While it's not known whether the legend is true, Lenski said, "it may be a convenient explanation for a Christian version of what happened at Lupercalia."