St. Valentine was a real, actual martyr. In fact there were 3 or 4 martyrs named Valentine. The only thing we know about any of them is that one of them was buried by the Via Flaminia on the 14th of February. Valentine’s Day was first celebrated in 496 AD.
The association with romantic love is due to Chaucer, who made poesy of Valentine’s Day as the day when birds mated. The idea caught on, and was such common currency in Shakespeare’s time that he used it in one of Ophelia’s mad babbles.
Valentines were mailed as early as the late 18th century, before there was any such thing as a “greeting card”. In fact, the greeting card industry appears to have come into existence to meet the need of mailing Valentines, not the other way around.
So everyone who’s ever complained about this holiday because they don’t have their flowers-and-chocolate trade pact set for the year are going to have to come up with something else.
At least, according to Wikipedia.