Now heavily commercialised and laden with expectation, the annual event was once a day where people earnestly showed their love and affection for another person.

The oldest surviving Valentine's poem was written by a prison-entrapped, pining lover: Charles, Duke of Orleans wrote it for his wife in 1415, confined in the Tower of London after being captured at the Battle of Agincourt.

However Valentine's Day was celebrated for centuries before that. From who the saint was to the best romantic gift ideas, here is everything you need to know about Valentine's Day.

When is Valentine's Day ?

The event falls on the same day each year. February 14 of course - which this year is a Thursday. Couples across the globe typically recognise the annual celebration by exchanging gifts, flowers and cards, although it it isn't a public holiday in every country.

While Valentine's Day is now heavily commercialised, the church originally decided to make the day a Christian celebration to honour St Valentine.

The feast of St Valentine of February 14 was first established in 496 by Pope Gelasius I, who included Valentine among all those "... whose names are justly reverenced among men, but whose acts are known only to God."

Who was St Valentine?

While the details of who St Valentine was are contested, one thing is agreed upon: he was martyred and buried on February 14 at the Roman cemetery on the Via Flaminia, the ancient road from Rome to Rimini.

However, the details we have of St Valentine could be of one saint or two conflated saints with the same name; this means there are many different biographies in circulation.

The most popular legend is that St Valentine - a priest from Rome - was arrested after secretly marrying Christian couples, who were being persecuted by Emperor Claudius II in the third century AD.

As helping Christians was considered a crime, St Valentine was imprisoned; while in jail he attempted to convert the emperor to Christianity and was condemned to death. He was beaten with stones and clubs, before being beheaded outside the Flaminian Gate.

According to some, while in prison St Valentine fell in love with the jailer’s daughter and sent her a love letter signed ‘from your Valentine’ on February 14, the day of his execution, as a goodbye.

Wearing a coronet made from flowers and with a stencilled inscription, St Valentine's skull now resides in the Chiesa di Santa Maria in Cosmedin, on Rome’s Piazza Bocca della Verità.

A casket containing a number of St Valentine's bones and a vial of his blood have been stored in the Whitefriar Street Church, in Dublin, since 1936, and couples regularly visit the religious shrine to ask him to watch over their lives.

Many couples preparing to marry also head to this church on February 14, the feast day of the saint, for a Blessing of the Rings in the presence of the reliquary.

What's Cupid got to do with it all?

Cupid is the god of desire, erotic love, attraction and affection. He is often portrayed as the son of the love goddess Venus and the war god Mars.

Cupid is also known in Latin as Amor ("Love"). His Greek counterpart is Eros and he is just one of the ancient symbols associated with St Valentine’s Day, along with the shape of a heart, doves, and the colours red and pink.

He is usually portrayed as a small winged figure with a bow and arrow which he uses to strike the hearts of people. People who fall in love are said to be ‘struck by Cupid’s arrow’.

Why is the heart associated with love ?

The heart was once associated with knowledge as well as feelings: Egyptians believed that the heart was the source of our memories, as well as our emotions. They placed so much value on the organ that they left it in people's bodies during mummification, while throwing all other organs, including the brain, away. And they weren't the only ones - Aristotle also believed that the heart was an organ of intellect.

This idea was widely accepted until Galen, a Roman physician, said the heart was more likely to be responsible for emotions than reason - apart from love, which was found in the liver.

As the influence of Christianity grew in the Middle Ages, so did the religion's pairing of the heart with love. 'Courtly love', where knights wooed women, became popular in the eleventh century and was tied to spiritual attainment.

It became popularised in lyric poems written by troubadours, such as William of Aquitaine, one of the first troubadour poets. Some say he was likely influenced by similar views on love in the Islamic world, which he came into contact with during the First Crusade.

In 1184, poet Andreas Capallenus referred to the organ as one of affection, writing ‘the pure love which binds together the hearts of two lovers with every feeling of delight’.

Around the same time, members of European families began to insist their hearts were buried separately from the rest of their bodies, in places that were special to them. In 1199 King Richard I of England had his heart buried in Rouen in Normandy and his body in Anjou, where his father was buried.

Over the centuries, the idea that the heart is linked to emotion has persisted and the two are now intrinsically linked.

When did Valentine's Day become so commercial?

It was during the middle of the 18th century that Valentine's Day started to take off in England, with lovers sending sweets and cards adorned with flowers, ribbons and images of cupids and birds.

Eventually huge numbers of printed cards replaced hand-written ones. In 1913, Hallmark Cards of Kansas City began mass producing Valentine's Day cards.

Now about a billion Valentine's Day cards are exchanged every year and it's the second largest seasonal card sending time of the year.

However, not all the cards are intended to be read: every year, thousands of letters addressed to Juliet are sent to Verona, where Shakespeare's fictional Romeo and Juliet lived.

What to write in a Valentine's Day card

What message will you be writing to your loved one this Valentine's Day?

If you're thinking of just putting "Happy Valentine's Day" and leaving it there - well, that's fine. Not all of us can be poets. But if you wanted to go for something a bit more elaborate, why not take inspiration from some of the greatest love letters ever written?

Plus, if you want to quote the modern greats, look no further than Telegraph Culture's collection of the best love poems ever written.

Why do some people leave anonymous cards?

This trend was started by the Victorians, who thought it was bad luck to sign Valentine's cards with their names.

The Victorians also started the rose-giving trend. They were the favourite flower of Venus, the Roman goddess of love, and have come to indicate passion and romance.

Nowadays, more than 50 million roses are given for Valentine's Day every year. There will of course be some people who do not receive any cards, flowers or gifts on Valentine's Day. In 2016, one teenager solved that problem by buying 900 carnations and giving them to out to all the girls at his school.

The Telegraph

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