Kumbh Mela is held every third year at one of the four places by rotation: Haridwar, Allahabad (Prayag), Nasik and Ujjain. Ardh (“Half”) Kumbh Mela is held at only two places, Haridwar and Allahabad, every sixth year. The rivers at these four places are: the Ganga at Haridwar, the confluence (Sangam) of the Ganga and the Yamuna and the mythical Saraswati at Allahabad, the Godawari at Nasik, and the Shipra at Ujjain.
The first written evidence of the Kumbha Mela can be found in the accounts of Chinese monk Xuanzang (formerly romanised as Hsuan Tsang) who visited India in 629-645 CE, during the reign of King Harshavardhana. However, similar observances date back many centuries, where the river festivals first started getting organised.
According to medieval Hindu theology, its origin is found in one of the most popular medieval puranas, the Bhagavata Purana. The Samudra manthan episode (Churning of the ocean of milk), is mentioned in the Bhagavata Purana, Vishnu Purana, the Mahabharata, and the Ramayana.
The account goes that the Devas had lost their strength by the curse of Durväsä Muni, and to regain it, they approached Lord Brahma and Lord Shiva. They directed all the demigods to Lord Vishnu and after praying to Lord Vishnu, he instructed them to churn the ocean of milk Ksheera Sagara (primordial ocean of milk) to receive amrita (the nectar of immortality). This required them to make a temporary agreement with their arch enemies, the Asuras, to work together with a promise of sharing the wealth 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 Devas and Asuras fought in the sky for the pot of amrita. It is believed that during the battle, Lord Vishnu (incarnated as Mohini-Mürti) flew away with the Kumbha of elixir spilling drops of amrita at four places: Allahabad (Prayag), Haridwar, Ujjain and Nashik
Royal Bath: Mauni Amavasya traditionally attracted the largest crowds, held here every 12 years. The day marked the second and the biggest Shahi Snan (royal bath) of this event, with 13 akharas taking to the Sangam.
This was the biggest bathing day in February at the ongoing Maha Kumbh Mela and probably the largest human gathering on a single day. Over 30 million devotees took holy dip on the occasion of Mauni Amavasya.