From National Geographic news this morning:
More than a Hallmark holiday, Valentine's Day, like Halloween, is rooted in pagan partying. (See "Halloween Facts: Costumes, History, Urban Legends, More.")
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.
However you choose to celebrate, enjoy the day!