In Hindu mythology, root of Kumbh Mela is in Samudra Manthan or Churning of Ocean. Samudra Manthan was a divine episode involving the Devta (Deva or God) and Asura (Demon) in search of amrita or nectar of immortality. Vedic literatures edify that, on the instruction of Lord Vishnu, once both Devtas and Asuras decided to churn the Milk Ocean to discover the celestial drink of immortality and share it mutually. However, Vishnu secretly told the Devtas that he would deprive the Asuras of the nectar.
To churn the ocean, a mountain Mandara was used as churning stick which was resting on the back of a Kurma (an epical tortoise, avatar of Lord Vishnu). While multiple headed King of Snakes Vasuki offered to be the churning rope. 54 Devtas and 54 Asuras were on either side of the rope pulling it to and fro to churn the ocean. The Asura were holding towards the head of Vasuki and Devtas were on the tail end.
As the Samudra Manthan progressed, Milk Ocean released fourteen precious articles or the chaturdash ratnas that includes – Halahala – deadly poison, Surabhi or Kamadhenu- the cow of plenty, Uchchaihsravas – the white flying horse, Airavat – the multi-tusked elephant of Lord Indra, Kaustava – the jewel worn by Lord Vishnu, Parijata – the celestial flowering tree, The Apsarasas – the nymphs of heaven, Lakshmi – Goddess of wealth and fortune, Varuni or Sura – Deity of wine, Chandra or the Moon, Kalpavriksha – the wish fulfilling tree, Sankha – the conch of Lord Vishnu, Sharanga – the powerful bow of Lord Vishnu, Dhanwantari – the physician of Devta and God of Ayurveda, Amrit or Amrita – the nectar of immortality in a golden Kumbha.
At the pinnacle of Samudra Manthan, strongly built Dhanvantari emerged carrying the jug (Kumbh) of nectar. The impatient Asuras, desiring to grab the Kumbh of Amrita, tried to snatch it away by force . The divine carrier flew with the pot and fight erupted. In other versions of the legend, Garuda took the pot and flew away. During that process, few drops of the nectar fell at four different places: Haridwar, Prayagraj (Allahabad), Trimbak (Nashik) and Ujjain where the kumbh Mela is celebrated after every 12 years. However, an Ardh Kumbh Mela is celebrated after 6 years (i.e. between the two Purna Kumbha Mela) in Allahabad and Haridwar. The celebration dates of this holy event at a particular place are calculated according to a special combination of zodiacal positions of Jupiter, the Sun and the Moon.
Kumbh Mela has high spiritual and religious importance in the life of Hindus and millions gather to take a holy dip in Divine River during the Kumbh to attain moksha.