Valentine's Day, or as it is more formally known, Saint Valentine's Day, is celebrated on 14th February each year. The Valentines that are commemorated or two Italian saints, Valentin or Valentnus, who share the saint's day of 14th February. The date is now when lovers declare their love by sending each other gifts and romantic cards. There's nothing especially romantic about the lives of the two original Valentines, they were both martyred for their faith, in Ad 197 and AD 269 respectively.
The early tradition of Valentine's Day was that it was the date that birds began to choose their mates, only later did the romance extend to the human population. The first reference in print to Valentine's Day is found in Geoffrey Chaucer's The parlement of foules [The Parliament of Fowls], circa 1381:
For this was on seynt Volantynys day Whan euery bryd comyth there to chese his make.
[For this was Saint Valentine's day, when every bird of every kind comes to this place to choose his mate.]
How the date of 14th February was selected isn't known. It may relate to the approximate date first anniversary of the engagement of King Richard II of England to Anne of Bohemia, which Chaucer's poem was written to honour. The couple were both 14 at the time of the engagement, which took place on 2nd May 1381, not on 14th February. The betrothal of young lovers sounds promising as a romantic story bit, in fact it wasn't. The marriage was purely a political contract between Anne's brother, King Wenceslas IV of Bohemia and the English government – the partners were unlikely to have met prior to the marriage.
The earliest known romantic valentine verse was written by Charles, Duke of Orleans to his wife in the 15th century:
Je suis desja d'amour tanné
Ma tres doulce Valentinée
[I am already sick of love,
My very gentle Valentine]