There are festivals and there are festivals indeed!

Have you heard of the surprising sexual celebratory ceremony taking place on the island of Java? You probably haven’t.

In the Indonesian tradition is a celebration called Pon. The revellers will hike up Gunung Kemukus, a hilltop shrine, found on the island of Java to take part in a holy ceremony to ensure good luck and fortune by having sex.

Yes, that’s right! Participants in the festival of Pon, have to spend the night atop the mountain and have ritualistic sex with a stranger if they want good luck and fortune to befall them in the future – even if both parties are married to other people.

What’s more is that tradition holds that this will only work if they have sex with the exact same person all seven times every 35th day throughout the year during Pon. Yes… these are not one-off encounters; the couples must complete the cycle if the ritual is to be considered complete. This means that they will have sex for the best part of a year.

The ritual itself involves offering prayers and flowers at the grave site of Pangeran Samodro – one of the sons of a Javanese king. Participants then must wash in a sacred spring and then find a stranger to do the horizontal tango with.

The Pon Festival is a paradox: “otherwise devout Muslims” – men and women – meet regularly on the same site for adulterous sex – pardon my use of ‘adulterous’ if it sounds offensive.

The relationship has all the components of a committed, caring relationship, though it is anything but.

For centuries, pilgrims have convened at the Gunung Kemukus, on the auspicious day of Jumat Pon (from which the Festival takes its name).

The shrine is believed to house the remains of a 16th-century prince called Pangeran Samudro, the son of a Javanese king; next to him are the remains of his stepmother, Nyai Ontrowulan.

Several versions of their story abound but it is believed that Samudro and Ontrowulan were forced to flee their home when Samudro’s father discovered their affair. The pair arrived in Solo only to be caught, mid-coitus, by villagers and killed.

Opinion is divided over what happened next: one re-telling states that before he died, Samudro sanctioned adulterous intercourse on Pon Friday. Another suggests that couples who commit a more scandalous act than Samudro and Ontrowulan’s will be rewarded with good fortune and that they must have sex on the grave to get it.

Whatever the real story, thousands of pilgrims converge at the site in the belief that sex will bring them good luck. Pilgrims come seeking success for their businesses or for personal gain to pay off debts.

The proceedings begin with early morning prayers.

Pilgrims then attend the grave of the prince and his star-crossed stepmother to lay flowers and pray again.

From here, they must cleanse themselves in one of the sacred springs, and only at this stage do they head off in search of a partner.

At sunset, the mountain is replete with pilgrims looking for sex.

The night is something of a reunion for returning couples.

Partners frequently exchange mobile numbers and addresses in order to find one another at the site,­ according to an SBS documentary, which means some couples stay in contact on non-ritual days.

Relationships occasionally form away from the mountain; even in the midst of this unusual tradition, people do fall in love but it is mostly unrequited as their intended is, well, married.

Some pilgrims tell their legal spouses that they are travelling to Gunung Kemukus. Most do not.

Traditionally, the sex is performed out in the open, and the couple will spend the remainder of the night together under the trees that dot the hillside.

In November 2014, the Central Java Governor Ganjar Pranowo moved to ban the ritual all together asking: “If you want to become rich, why should you engage in promiscuous sex?”

It’s a pertinent question but the Pon Festival is in little danger of being expunged.

Gunung Kemukus remains a site of religious significance with pilgrims travelling hundreds of miles to pray at Prince Samudro’s grave.

It is also highly lucrative: the local government charges the vendors that set up shop on the mountain and pilgrims are charged a hefty fee to enter.

It means that the sex trade will continue to thrive but things are improving.