In the USA alone, Valentine's Day spending is predicted to hit 21 billion next week in gifts for partners, friends, pets, and more. That is more than the gross national product of Cambodia.

How did it all begin? There is no proven beginning to Valentine's Day, but here are a few contenders.

First, the heroic priest, Valentinus. After Emperor Claudius II banned marriage in the belief that single men made better soldiers, Valentinus began to perform marriages for young lovers in secret. His execution, on 14 February, marked the beginning of Valentine's Day.

But this is where is gets confusing. BollandistA French order, originally part of the Jesuits. monks discovered another Valentinus, a priest from the same century. He was also believed to be executed on 14 February.

One of the two Valentinuses is thought to have fallen in love while imprisoned, and sent his love the very first Valentine's letter signed, "from your Valentine".

What is Valentine's Day? Valentine's Day is an annual celebration in which millions of people around the world celebrate their love for one another.

And nowadays, it isn't only limited to humans. It is thought that, last year, people spent over $880 million (676m) on Valentine's gifts for their pets. You could feed over a million children in Africa for a day, for that amount.

Any other odd theories? Yes. One of the oldest origin stories revolves around a paganSpiritual beliefs other than those of the main or recognised religions. Often used in a historical context. festival of fertilityThe ability to have a child. called Lupercalia, held on 15 February. Women would pin the name of their suitor on their sleeve - a possible origin of the common phrase: "to wear your heart on your sleeve".

In a religious power play, Pope Gelasius outlawed Lupercalia and declared 14 February St Valentine's day. "It was a little more of a drunken revel," historian Noel Lenski explains, "but the Christians put clothes back on it."

But is it all based around religion? Not at all. A valid argument can be made for Geoffrey Chaucer being the origin of the holiday as we know it today. Jack B OruchA distinguished American historian, formerly at the University of Kansas. argues that, in Parliament of FoulesA short poem by Geoffrey Chaucer, made up of approximately 700 lines., he is the first to recognise Valentine's Day as a romantic celebration.

In nature, February used to mark the beginning of spring, and the beginning of mating season for birds. Love was literally flying through the air.

How has Valentine's Day changed? Blame the Industrial RevolutionA period of sudden, rapid industrialisation that transforms a country from an agricultural to an industrial economy.. Valentine's cards could be printed for the masses and, along with cheaper postage, this led to a rise in the holiday's popularity across the globe. At last, people had a mechanically printed card to show their love: how on Earth did they manage before?!

And the commercial business of Valentine's Day continues to grow. The US National Retail Federation (NRF) estimates that this year people are going to spend $27 billion (21bn) in the US alone on cards, chocolates, and flowers among other special gifts. That's $7 billion (5bn) more than last year.

Do all countries celebrate Valentine's Day? Although not all countries call it by this name, a great number dedicate a day of the year to the celebration of love, though not all of it is romantic. Estonia, for example, celebrates Sobrapaev on 14 February, which is a day of friendship.

At the other end of the spectrum is the Philippines, whose government sponsors mass weddings where hundreds of couples say "I do" at the same time. Maybe not the most unique experience, but it is paid for by the state.

But just why is it so popular? As a holiday that was given new value by the likes of Hershey's and Hallmark, Valentine's Day is not necessarily increasing in popularity. In fact, although the spending for the holiday has increased, the amount of people participating is slowing down.

What if I hate the whole idea? How about looking into the rise of a new phenomenon, Galentine's Day?

This unofficial feast day takes place the day before on 13 February, celebrating female independence in the modern age, and total freedom from the romantic obligations of Valentine's Day.