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 legalized Christianity in the Roman Empire. Read more