From The Writer's Almanac.
The romantic overtone of the holiday is in commemoration of St. Valentine, a Roman priest who was martyred on February 14 in 269 A.D. ... The tradition of exchanging love notes on Valentine's Day originates from the martyr Valentine himself. The legend maintains that due to a shortage of enlistments, Emperor Claudius II forbade single men to get married in an effort to bolster his struggling army. Seeing this act as a grave injustice, Valentine performed clandestine wedding rituals in defiance of the emperor. Valentine was discovered, imprisoned, and sentenced to death by beheading. While awaiting his fate in his cell, it is believed that Valentine fell in love with the daughter of a prison guard, who would come and visit him. On the day of his death, Valentine left a note for the young woman professing his undying devotion signed "Love from your Valentine."