Lupercalia survived the initial rise of Christianity and but was outlawed—as it was deemed “un-Christian”--at the end of the 5th century, when Pope Gelasius declared February 14 St. Valentine's Day. It was not until much later, however, that the day became definitively associated with love. During the Middle Ages, it was commonly believed in France and England that February 14 was the beginning of birds' mating season, which added to the idea that the middle of Valentine's Day should be a day for romance.
Valentine greetings were popular as far back as the Middle Ages, though written Valentine's didn't begin to appear until after 1400. The oldest known valentine still in existence today was a poem written in 1415 by Charles, Duke of Orleans, to his wife while he was imprisoned in the Tower of London following his capture at the Battle of Agincourt. (The greeting is now part of the manuscript collection of the British Library in London, England.) Several years later, it is believed that King Henry V hired a writer named John Lydgate to compose a valentine note to Catherine of Valois.