Until fairly recently, St. Valentine actually was a full-fledged Catholic saint, albeit an obscure one, and Feb. 14 was his time-honored feast day. Because St. Valentine was believed to have been an early Christian martyr, the priests saying Mass on that day wore red vestments symbolizing his martyr's blood. The red of the vestments became the color of St. Valentine's Day. And because St. Valentine was thought to have had something vaguely to do with love (no one was really sure what), red hearts, symbols of love, became ubiquitous on St. Valentine's Day, especially after the greeting card industry discovered him during the 1840s and started marketing commercial "valentines"--the lacy greeting cards bearing Cupids and romantic verses that we know so well.
Then, after the Second Vatican Council, the Catholic Church, embarrassed by the presence of saints on its calendar who might never have existed, booted the already shadowy St. Valentine from his Feb. 14 slot. Into his place, the Church moved saints Cyril and Methodius, two bishop-brothers who had brought Christianity to the Slavs of Eastern Europe during the ninth century. There was never any doubt as to saints Cyril's and Methodius's existence, or of their heroic virtue. The brothers braved years of political and religious persecution as they preached the Gospel. St. Methodius translated the Bible and other Christian works into Slavonic, which is still the liturgical language of many Eastern churches, and St. Cyril is credited with the invention of the Cyrillic alphabet used by Bulgarians, Russians, and Serbs. The priests saying Mass on Feb. 14 switched to white vestments in honor of the brothers' holiness.
It does seem as though there was a real St. Valentine who was martyred during Roman times. In fact, there seem to have been two, perhaps three saints Valentine. Valentine--or Valentinus, derived from the Latin adjective valens, meaning strong and powerful--was a popular man's name during the classical period. The best-known St. Valentine was believed to have been a Roman priest beheaded for his Christian faith toward the end of the third century, under the reign of the Emperor Claudius the Goth. A church dedicated to him (or perhaps one of the other two saints of the same name) was erected in Rome about a century after his presumed death.
Other than that, St. Valentine never acquired much of a cult following, as did the more popular medieval saints. The Mass said in his honor was a generic Mass for martyrs into which the priest inserted the words "blessed Valentine" here and there. Even his traditional saint's legend was short and fairly generic: Valentine refuses to worship the pagan gods, Valentine loses his head. No appalling tortures, no dramatic visions, and nothing remotely associated with romance.
St. Valentine became associated with lovers because his feast day fell on the day before the licentious Roman fertility feast of Lupercalia on Feb. 15. Lupercalia was a sort of Spring Break Week a month early. After an early-morning sacrifice of goats and dogs to Faunus, the god of merry-making, a band of young men, naked except for loincloths made from the skins of the sacrificed animals, ran along the boundaries of Rome, hitting those whom they met, especially women, with strips of the skins. After that, there was quite a bit of drinking, dancing, and what is now called "hooking up." As it did with Saturnalia (now Christmas) and Halloween, the Church did not try to suppress the holiday completely, but Christianized it by discouraging the improprieties and linking it to the saint believed to have been martyred on its eve.