Maha Kumbh: The Holy Dip

The Kumbh Mela is the world's largest gathering of people for a religious purpose.

The Festival is every year, however the main one is every 12 years, when the planets are in maximum alignment. Based on an ancient Hindu myth, and has been going on since at least the 5th century. It is located in central India, just outside the town of Allahabad, on the dry riverbed of the Ganges River.

http://kumbhmelaallahabad.gov.in/english/kumbh_at_glance.html

http://www.kumbhmela.org/