Hundreds of millions of Hindus will gather this week for what is expected to be the world’s largest gathering of people, where a staggering number of devotees, tourists, politicians and celebrities take a holy dip at the confluence of two holy rivers in India.

The religious festival, called the Maha Kumbh Mela, is held every 12 years on the banks of the Ganges and Yamuna rivers in the northern Indian city of Prayagraj. This year, officials expect up to 400 million people – more than the population of the United States – to visit the site in Uttar Pradesh state over the next six weeks.

As a major display of Hinduism, the event has recently become an important political event with the rise of Hindu nationalism, supported by Prime Minister Narendra Modi’s right-wing political party. It’s also a huge logistical undertaking for government officials working to prevent incidents like stampedes and the spread of disease.

What is the Maha Kumbh Mela?

The Maha Kumbh Mela, or “great festival of the holy pitcher”, is the largest religious ceremony in the world. Based on a Hindu legend in which demons and gods battle over a pitcher bearing the nectar of immortality, the centuries-old ceremony centers on a series of holy baths that Hindus say cleanse their sins.

The holy bath is preceded by processions in which people sing and dance in lively clothes, in lavishly decorated chariots and holding ceremonial spears, tridents and swords. To participate, people travel from all over India and the world to the confluence of the Ganges and Yamuna rivers, a sacred site also said to be the endpoint of the mythical third river, the Saraswati.

The timing of the festival, which this year ends on February 26, is based on the astrological alignment of the Sun, Moon and the planet Jupiter, which takes about 12 years to orbit the Sun. Smaller versions of the festival are held in one of three Indian cities – Haridwar, Nashik and Ujjain – about every three years.

How big is the festival?

The scale of the Maha Kumbh Mela is staggering. The last one, in 2013, drew 120 million people to Prayagraj, according to government estimates. The Middle Festival of 2019, although less significant in religious terms, attracted 240 million people.

This year, the city, home to about 6 million people, is preparing to welcome 300 to 400 million people, government officials said. In preparation, the state has built a temporary camp on an area of ​​10,000 hectares, with tens of thousands of tents and bathrooms, roads, parking lots, water and electricity infrastructure, and thousands of security cameras and drones.

Many of these preparations – likely to make this the most expensive Maha Kumbh Mela yet, at around $800 million – are intended to prevent deadly stampede and disease epidemics, which occurred at previous festivals. The event is also expected to generate billions of dollars in revenue for the state government, officials said.

To accommodate bathers, the government has also installed a platform made of sandbags along a seven-mile stretch of the Ganges riverbank. On Monday and Tuesday, millions of pilgrims poured into the river on those steps in the cold morning mist, praying for luck, health and prosperity.

What is the significance of the festival today?

The Maha Kumbh Mela has always been an important symbol of Hinduism, although it has not usually been politicized until the recent rise of the idea of ​​India as a Hindu nation. This year’s festival is the first since Mr. Modi’s Hindu nationalist political party, the BJP, became the country’s ruling party 11 years ago.

“It would be interesting to see if Prime Minister Modi leaves,” said Arati Jerath, a New Delhi-based political analyst. “That should be the greatest and most auspicious time for bathing in the Ganges.”

Yogi Adityanath, the chief minister of Uttar Pradesh who is also a hardline Hindu priest, has changed the name of the host city of the 2018 festival to Prayagraj from Allahabad. The move, part of a wave of changes brought in by the BJP, replaced the Muslim name given by the 16th-century Mughal emperor Akbar with a name referring to a Hindu pilgrimage site.

In 2019, when the general elections were held in India, the Kumbh Mela presented a a great political opportunity Mr. Modi and his party to appeal to a receptive audience of millions. Mr. Modi won that election.

The next general elections are far away this time, they are scheduled for 2029. But Mr. Modi, who won with smaller margin while his party suffered losses in last year’s vote, he put himself on promotional posters for the festival across the country and called it the embodiment of “India’s timeless spiritual heritage” on social mediaconnecting the spiritual event with the country’s national identity.

“The BJP hopes to use this to consolidate its Hindu nationalist base,” Ms Jerath said. But she added that it is not clear whether this will necessarily bring the party more votes. “I don’t know if it works or not, but it certainly helps bring the BJP one step closer to its goal of making India a Hindu nation.”