The Hindu festival of the Kumbh Mela commemorates a celestial battle between gods and demons. Because the battle lasted for 12 divine days, a full Kumbh Mela is celebrated every 12 years. The time in between this is split up by an Ardh Kumbh Mela which occurs every six years. The most recent Maha Kumbh Mela which occurred in 2001 drew an astounding 70 million pilgrims to the Sangam (where the Ganges and Yamuna rivers meet).
Photo caption: The 2001 Kumbh Mela Credit: Wikipedia user Devinasch / CC BY-SA 3.0
Sources:
» BBC.. "Millions bathe at Hindu festival" January 3, 2007.
» The Times (UK). "Kumbh Mela: the largest pilgrimage." March 22, 2008.