Ujjain is the holiest city in the Mythology blessed by lord Shiva the Mahakaal. The simhasta Kumbhaa mela is a very pious and celebrated festival in Ujjain as because of the seat of Mahakaal making the manthan possible by consuming the deadly poison which poured out after the churning.
During the Kumbhaa mela astronomical positions are similar which had led to the Samudra manthan in the ancient period with the nectar falling at four different places on earth. The cosmic energy felt during the festival is same which in earlier days the devotees use to feel and for the same all devotees during this festival gather for a dip in the holy waters. It is said that the waters are so pure that all sins of you and even of your ancestors gets washed.
Legends tell from bygone days of the universe that the demigods and the demons conjointly produced the nectar of immortality. The demigods as cursed and crippled of fear, the task being too sturdy made arrangements with the demons to complete the task by agreeing to share the nectar of immortality in half but demons never got the nectar even after fierce fighting with the demigods. The poison coming out of the churning was gulped by Lord Shiva.
A prevalent legend is the demigods went behind the demons for 12 days and 12 nights, and the drops of elixir of immortality fell at four places. The four places then believed to have acquired mystical powers because 12 days of gods and 12 years of humans are same.so this festival is celebrated every 12 years at river Godavari in Nasik, river Kshipra in Ujjain, river Ganges in Haridwar, and Sangam of Ganges in Allahabad,
The time line for Kumbha mela in Ujjain goes as—
1980, 1992, 2004, 2016