Millions of people will bathe in India's sacred Ganges and Yamuna rivers as part of the Hindu festival of Kumbh Mela. It's the biggest human gathering in the world with over a million people expected. Hindus believe the water from the river will rid them of sin and save them from any future evil. The location of the festival is chosen solely by the position of the sun, moon and Jupiter according to Hindu astrology. A Maha Kumbh mela is held once every 12 years.