The first recorded Valentine was sent February 1415 by the English duke of Orleans. He sent of love letter to his wife from his jail cell in the Tower of London after the Battle of Agincourt. It is currently on display in the British Museum. Fact source...
The popular medieval folk belief that birds choose their mates on February 14 made doves a favorite symbol for Valentine cards. The dove was sacred to Venus and other love deities and was known for choosing a lifelong mate. Fact source...
The saying “wearing your heart on your sleeve” is from the Middle Ages. Boys at this time would draw names of girls to see who would be their “Valentine” and then wear the name pinned on their sleeve for a week. Fact source...