Kumbh Mela - (Festivals in India)

Bakri-Id - Festival of Sacrifice - Muslim
Buddha Jayanti - Buddha's Birthday - Buddhist/Hindu
Diwali - Festival of Light - Hindu
Dussehra or Dusshera - Lord Ram - Hindu
Guru Nanaks Birthday - Sikh
Holi - Festival of Colour - Hindu
Kumbh Mela - Festival of the Pot - Hindu
Meelad-Id - The prophet Muhammed's birth and death - Muslim
Onam - South Indian celebration of good over evil - Hindu
Rakhi - Sibling closenes - Hindu/Many

Kumbha Mela

The largest religious gathering in the world. According to astrologers, the 'Kumbh Fair' takes
place when the planet Jupiter enters Aquarius and the Sun enters Aries.
Kumbha (Kumbha means pot) Mela (means fair) is a sacred Hindu pilgrimage that takes place at the following four locations of India:
Prayag, Allahabad (UP) at the confluence of three holy rivers - Ganga (Ganges), Yamuna and the subterranean Saraswati
Haridwar (UP) where the river Ganga enters the plains from Himalayas
Ujjain (in Madhya Pradesh), on the banks of Shipra river, and
Nasik (in Maharashtra) on the banks of Godavari river.

The pilgrimage occurs four times every twelve years, once at each of the four locations. Each twelve-year cycle includes the Maha (great) Kumbha Mela at Prayag, attended by millions of people, making it the largest pilgrimage gathering around the world. A once in a life time event.

Kumbh Mela from space - New Scientist

About Kumbh:-

Kumbh's history originates from the beginning of the Creation. All devtas (demi-gods) were under the influence of a curse that made them weak and cowardly, Brahma, (the creator God) advised them to churn the ocean for Amrit (Nectar), which would make them immortal. The devtas sough the help of demons for the purpose. Gods and demons made a temporary pact to work together in obtaining amrita from the Milky Ocean, and to share this equally. The Mandara Mountain was used as a churning rod, and Vasuki, the king of serpents, became the rope for churning. With the demigods at Vasuki's tail and the demons at his head, they churned the ocean for a 1,000 years. However, Dhanwantari, the divine healer, appeared with the "Kumbh" containing nectar in his palms, when the Kumbha (pot) containing the amrita appeared, the demons ran away with the pot and were chased by the gods. By their joint churning Amrit was one of the resultants, found in the last. A battle ensured. 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. It is said that during the battle, drops of amrita fell on to four places : Prayag, Haridwar, Ujjain and Nasik . Kumbha mela is observed at these four locations where the nectar fell.