Saint Valentine and his day.

Saint Valentine (Italian: San Valentino; Latin: Valentinus) was a 3rd-century Roman saint, commemorated in Western Christianity on February 14 and in Eastern Orthodoxy on July 6. From the High Middle Ages, his Saints' Day has been associated with a tradition of courtly love. He is also a patron saint of Terni (a city in the southern portion of the region of Umbria in central Italy), epilepsy and beekeepers.

He was born c.226 and died c.269 (aged 42–43)

Saint Valentine was a clergyman – either a priest or a bishop – in the Roman Empire who ministered to persecuted Christians. He was martyred and his body was buried at a Christian cemetery on the Via Flaminia on February 14, which has been observed as the Feast of Saint Valentine (Saint Valentine's Day) since at least the eighth century.

The Feast of Saint Valentine was established by Pope Gelasius I in 496 to be celebrated on February 14 in honour of Saint Valentine of Rome.

The day became associated with romantic love in the 14th and 15th centuries when notions of courtly love flourished, apparently by association with the "lovebirds" (the mating rituals of birds) of early spring. In 18th-century England, it grew into an occasion in which couples 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. Since the 19th century, handwritten valentines have given way to mass-produced greeting cards. 

The first recorded association of Valentine's Day with romantic love is believed to be in the Parliament of Fowls (1382) by Geoffrey Chaucer:

As written in Middle English:

"For this was on seynt Valentynes day

Whan every foul cometh there to chese his make

Of every kynde that men thynke may

And that so huge a noyse gan they make

That erthe, and eyr, and tre, and every lake

So ful was, that unethe was there space

For me to stonde, so ful was al the place."

Modern English translation:

"For this was on Saint Valentine's Day

When every bird comes there to choose his match

Of every kind that men may think of

And that so huge a noise they began to make

That earth and air and tree and every lake

Was so full, that not easily was there space

For me to stand—so full was all the place."

And Shakespeare of course:

"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 5

There are also an increasing number of references to the day from Chaucer onwards.

In 1797, a British publisher issued The Young Man's Valentine Writer, which contained scores of suggested sentimental verses for the young lover unable to compose his own. Printers had already begun producing a limited number of cards with verses and sketches, called "mechanical valentines." Paper Valentines became so popular in England in the early 19th century that they were assembled in factories. Fancy Valentines were made with real lace and ribbons, with paper lace introduced in the mid-19th century. In 1835, 60,000 Valentine cards were sent by post in the United Kingdom, despite postage being expensive.

In 1868, the British chocolate company Cadbury created Fancy Boxes – a decorated box of chocolates – in the shape of a heart for Valentine's Day. Boxes of filled chocolates quickly became associated with the holiday.

Since the 19th century, handwritten notes have given way to mass-produced greeting cards. In the UK, just under half of the population spend money on their Valentines, and around £1.9 billion was spent in 2015 on cards, flowers, chocolates and other gifts. The mid-19th century Valentine's Day trade was a harbinger of further commercialized holidays in the U.S. to follow.

The Valentines Day tradition developed in early modern England and spread throughout the English-speaking world in the 19th century.

In the 20th Century the 'Americanised' version spread throughout much of the rest of the world.