Kumbh Mela is a major Indian festival held every three years in one of four different cities, the largest of which happens in Allahabad every twelve years. Hindus gather at the confluence of the Godavari and Ganges Rivers where bathing in the water to purify oneself is thought to be auspicious and powerful. Kumbh Mela first played out in history sometime between 629 and 645 CE. Huan Tsang, a traveler from China, wrote about the festival after encountering it on his journey. Kumbh Mela is also mentioned in the Bhagavata Purana, one of the best known medieval puranas (medieval texts).
The timing of Kumbh Mela sets it apart from the many other Indian religious festivals and celebrations. Each festival is celebrated at a different location which depends on the planetary positions of both the Sun and Jupiter (Brahaspati); once the Sun and Jupiter are the zodiac Simha Rashi, or Leo, the festival happens in Trimbakeshwar, Nashik; the festival is in Haridwar when the Sun is in Mesha Rashi, or Aries; when Jupiter is in Vrishabha Rashi or Taurus and the Sun is in Makar Rashi (Capricorn) then Kumbh Mela is held in Prayag; and finally the festival is celebrated in Ujjain when the Sun and Jupiter are simultaneously in Vrishchik Rashi which is Scorpio. All of these dates are determined in advance based on a unique combination of positions of the Sun, Moon, and Jupiter within the zodiac.
The main festival events of Kumbh Mela are the bathing days, where pilgrims numbering in the millions gather to bathe themselves in the sacred Ganges River. During this time there are myriad religious rituals, processions of holy men called Sadhus (many of which are yogis), unusual vendors, and many different nomadic tribes who attend. Sadhus are an integral part of the ceremonies and celebrations as they are so exalted. They are focused solely on achieving a stage in life called "moksa," which translates to "liberation." This is the fourth and also the final stage or asrama, in life for Hindus.
Top image: Yosarian (wikipedia)