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.. By the middle of the 18th, it was common for friends and lovers of all social classes to exchange small tokens of affection or handwritten notes, and by 1900 printed cards began to replace written letters due to improvements in printing technology.  All found at http://www.history.com/topics/valentines-day.