Every 12 years, Prayagraj hosts the Purna Kumbh Mela, or the great Kumbh Mela, the world's largest pilgrimage that brings together several million people. At the 2001 Purna Kumbh Mela, 70 million people visited the banks of the Ganges ove three weeks, including 40 million in a single day. The Maha Kumbh Mela is held in Prayagraj every 144 years, after exactly 12 Purna Kumbh Melas, as is the case for the exceptional 2025 edition, for which record numbers of pilgrims are expected.

Legend has it that for 12 divine days and nights, the equivalent of 12 human years, gods and demons battle in the skies to possess the jug of amrita, the drink of eternity. During the battle, drops of amrita fall in four places: Prayagraj, Hardwar, Ujjain and Nashik, cities in which the Kumbh Mela festivals are held every 12 years. The flow of the Ganges symbolizes amrita, and celebrations take place around the river.

The main event of the festival is the immersion in the Ganges, which is meant to wash away all the sins of 88 generations on certain dates defined by astrological calculations. Sadhus, sacred men, bathe first, usually naked and in pairs, after which they cover themselves with ashes. Pilgrims come next. A temporary city has been built on a 4,000-hectare site to accommodate the faithful. It houses 160,000 tents with carpets, beds, sanitary facilities, 150,000 toilets, 68,000 lampposts and 25,000 garbage cans. Some 40,000 police officers have also been stationed. The government has invested enormous resources in communication to promote this festival on the borderline between the religious and the political, the spiritual and the business.