How Did Valentine’s Day Start?

Valentine's Day, also called Saint Valentine's Day or the Feast of Saint Valentine, is celebrated annually on February 14.

Valentine's Day customs – sending greeting cards, offering confectionary and presenting flowers – developed in early modern England and spread throughout the English-speaking world in the 19th century. In the later 20th and early 21st centuries, these customs spread to other countries (source Wikipedia) but how did Valentine’s Day start?