Shakespeare refers to Valentine's Day in Hamlet (Act 4, Scene 5) when Ophelia sings:

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.