Valentine's Day took the place of Lupercalia (ancient Roman festival of fertility) - Writer's Almanac

from Minnesota Public Radio - American Public Radio - Garrison Keillor:

Today is Valentine's Day, a big day for greeting card and candy sales, which goes back more than 1,500 years to the Feast of St. Valentine established in the fifth century, though nobody is sure exactly which of the many martyred Valentines it is the feast day of.
The ancient Romans had a fertility festival celebrated at mid-February of every year. The festival was called Lupercalia in honor of Lupa, the wolf who was said to have suckled Romulus and Remus, who went on to found the city of Rome. Lupercalia was a pagan fertility festival celebrated with sacrifices of goats and dogs, with milk and wool and blood. Young men would cut strips from the skins of the goats then strip naked and run through the city in groups, where young women would line up to be spanked with the switches, believing it would improve their fertility. Lupercalia was still wildly popular long after the Roman Empire was officially Christian, and it's not difficult to see why the Church would have wished to have a different sort of holiday take its place.
[entry in Writer's Almanac dot-org for 2/15/15]