Kumbh Mela

Kumbh Mela is a mass hindu piligrim.

its celebrated in every 4 to 12 yrs of intervals,depend upon places means 6 yrs at haridwar and prayag known as ardh[half] kumbh held at at four places (Prayag (Allahabad), Haridwar, Ujjain, and Nashik).
maha [great] Kumbh  Mela which comes after 12 'Purna Kumbh Melas', or 144 years, is held at Allahabad.
During this Kumbh Mela, which is considered to be the most important Hindu spiritual gathering, thousands of sadhus and pilgrims come from all over India and abroad. The most important features of this Mela are furious and exotic Naga Sadhus covered in ash, matted hairs and are known as preserver of faith.

 

When?
It is celebrated at different locations depending on the position of the planet of Bruhaspati (Jupiter) and the sun.When Jupiter and the sun are in the zodiac sign Leo (Simha Rashi) it is held in Trimbakeshwar, Nashik; when the sun is in Aries (Mesha Rashi) it is celebrated at Haradwar; when Jupiter is in Taurus (Vrishabha Rashi ) and the sun is in Capricorn (Makar Rashi) Kumbha Mela is celebrated at Prayag; and Jupiter and the sun are in Scorpio (Vrishchik Rashi) the Mela is celebrated at Ujjain

Ritual

 The major event of the festival is ritual bathing at the banks of the river in whichever town it is being held.Nasik has registered maximum visitors amounted nearly to 75 million. Other activities include religious discussions, devotional singing, mass feeding of holy men and women and the poor, and religious assemblies where doctrines are debated and standardized. Kumbh Mela is the most sacred of all the pilgrimages. Thousands of holy men and women attend, and the auspiciousness of the festival is in part attributable to this. The sadhus are seen clad in saffron sheets with ashes and powder dabbed on their skin per the requirements of ancient traditions. Some, called naga sanyasis, may not wear any clothes even in severe winter

The Mythology
 Kumbh's history is very old and originates from dates back to the time when Amrita Kalasha (pot of nectar of immortality) was recovered from Samudramanthan (the churning of the primordial sea). During this churning process, the Mandara Mountain was used as a churning rod, and Vasuki, the king of serpents, acted as the rope for churning. In this entire churning process, Dhanwantari, the divine healer, appeared with the "Kumbh" containing nectar in his palms, when the Kumbha (pot) containing the Amrita Kalasha appeared. After this a tense war between Devtas (Gods) and Asuras (Demons) ensued.

For twelve days and twelve nights (equivalent to twelve human years) the gods and demons fought in the sky for the possession of this pot of amrita. And it is said that during the battle, drops of amrita fell on to four places: Allahabad, Haridwar, Ujjain and Nasik. To commemorate this holy event of the Amrita Kalasha Kumbh is celebrated at these four places.

see official site for maha kumbh mela in 2013 n other more info
http://www.kumbhmela.org/KM10home.html

see this nice video of 35 min for more n full info
credit for this video/documentry is LifeBlissFoundation