The observance of Kumbha Mela dates back many centuries in ancient India, to the Vedic period, where the river festivals first started getting organized. In Hindu mythology, its origin is found the one of the popular creation myths and the Hindu theories on evolution, the Samudra manthan episode (Churning of the ocean of milk), which finds mention in the Shrimad Bhagavatam, Vishnu Purana, the Mahabharata and the Ramayana.

The Gods had lost their strength, and to regain it, they thought of churning the Ksheera Sagara (Primordial Ocean of milk) for amrit (the nectar of immortality), this required them to make a temporary agreement with their arch enemies, the demons or Asuras to work together, with a promise of sharing the nectar equally thereafter. However, when the Kumbha (urn) containing the amrita appeared, a fight ensued. For twelve days and twelve nights (equivalent to twelve human years) the gods and demons fought in the sky for the pot of amrita. It is believed that during the battle, the celestial bird, Garuda the vehicle of Vishnu flew away with the Kumbha of elixir, and that is when drops of amrita fell at four places on earth: Prayag, Haridwar, Ujjain and Nashik, and that is where the Kumbha Mela is observed every twelve years.

Learn more about Kumbha Mela from the Puranas here: Ganga and Amrita.