When did it begin?
According to Wiki the day was first associated with romantic love in the circle of Geoffrey Chaucer in the High Middle Ages, when the tradition of courtly love flourished. In 18th-century England, it evolved into an occasion in which lovers expressed their love for each other by presenting flowers, offering confectionery, and sending greeting cards (known as "valentines"). Valentine's Day symbols that are used today include the heart-shaped outline, doves, and the figure of the winged Cupid.
Valentine's Day is mentioned ruefully by Ophelia in Hamlet (1600–1601):
To-morrow is Saint Valentine's day,
All in the morning betime,
And I a maid at your window,
To be your Valentine.
Then up he rose, and donn'd his clothes,
And dupp'd the chamber-door;
Let in the maid, that out a maid
Never departed more.—William Shakespeare, Hamlet, Act IV, Scene 5Ahhh, how sweet it is to be in love.