Mankind's largest religious festival is related to the mythological story of the creation of the universe. Once the perpetual struggle between Gods and demons  - between good and evil – reached a stage, where the Gods were on the verge of losing. The creator Lord Brahma told them that from churning the Ocean of Milk they could produce the nectar of immortality called as Amrit. However, for that they needed the help of the demons. So an armistice between the fighting parties was concluded and they promised to share all that was won by the churning of the Milk Ocean equally. However, when the pot (kumbh) filled with the nectar of immortality appeared they started fighting each other for 12 days and nights (corresponding to 12 human years). Garuda, the celestial bird and vehicle of Lord Vishnu snatched the nectar pot and flew away chased by the demons. During the chase some drops of the nectar of immortality fell on four  places on earth. And these are the holy places where the Kumbh Mela is celebrated nowadays every 12 years – Allahabad, Haridwar, Nasik and Ujjain.
The Tirtharaja – King of Holy Places – among them is the Sangam  (confluence) of the sacred rivers Ganga, Jamuna and Saraswati in Allahabad. Here the world's biggest religious gathering will be held in January and February 2013 again. Thousands of sadus and millions of devotees will take a ritual bath to cleanse themselves of all sins. 
Although maybe not as old as the universe the Kumbh Mela was already celebrated 1400 years ago according to a report by  the Chinese traveller Huan Tsang.