The romantic nature of February 14 seems to have been attached to the date long after it was recognized as the feast day of St. Valentine. Still, several centuries before pre-printed, mass-produced greeting cards existed, men and women sent notes, tokens, and cards to their loves on St. Valentine’s Day. The tradition certainly was in place by the late 15th century, with French and English literature indicating the practice dated at least to the 14th century. However it is less certain exactly how St. Valentine’s Day came to be recognized as the romantic holiday it is today. There are multiple ideas on how the custom evolved.
The legend that describes St. Valentine’s imprisonment mentions a letter he sent to the Jailer Asterius’s daughter on the eve before he was executed. According to the legend, the farewell message was affectionately signed “From Your Valentine,” a phrase now popular on Valentine greeting cards everywhere.
Read more about the history of St. Valentine's Day, as well as the saint himself, here.