The King who organized Kumbh Mela at Prayagraj

Xuanzang mentions that Emperor Shiladitya (identified with Harsha) distributed his wealth among the public once every five years; his treasury was then replenished by his vassals. He describes such a ritual at a site located at the confluence of two rivers, in the kingdom of Po-lo-ye-kia (identified with Prayaga).

He also mentions that many hundreds take a bath at the confluence of two rivers, to wash away their sins. According to some scholars, this is earliest surviving historical account of the Kumbh Mela or its predecessor.

However, Australian researcher Kama Maclean notes that the Xuanzang reference is about an event that happened every 5 years (and not 12 years), and might have been a Buddhist celebration (since, according to Xuanzang, Harsha was a Buddhist emperor).

As per history, the Prayagwal Brahmin priests of Prayag adapted their annual Magh Mela to Kumbh legend for increasing the significance of their tirtha. 

Tradition ascribes the Kumbh Mela’s origin to the 8th-century philosopher Shankara, who instituted regular gatherings of learned ascetics for discussion and debate. The founding myth of the Kumbh Mela—attributed to the Puranas (collections of myth and legend)—recounts how the gods and demons fought over the pot (kumbha) of amrita, the elixir of immortality produced by their joint churning of the milky ocean. During the struggle, drops of the elixir fell on the Kumbh Mela’s four earthly sites, and the rivers are believed to turn back into that primordial nectar at the climactic moment of each, giving pilgrims the chance to bathe in the essence of purity, auspiciousness, and immortality. The term Kumbh comes from this mythic pot of elixir, but it is also the Hindi name for Aquarius, the sign of the zodiac in which Jupiter resides during the Haridwar Mela.