Kumbh mela is celebrated when Jupiter transits sidereal Kumbha rasi (Aquarius).Jupiter takes about 11.5 to 12 years to complete one round of zodiac and come to the same position again , hence Kumbh mela is held approximately every 12 years and ardh Kumbha is held when Jupiter transits the exactly opposite sign sidereal Simha or Leo.This is also held after every 11.5 to 12 years apart but approximately six years apart from Kumbha.

The Purna (complete) Kumbh takes place at four places (Prayag (Allahabad), Haridwar, Ujjain, and Nashik) every twelve years,[1] while the Ardh Kumbh Mela is celebrated every six years at Haridwar and Prayag.[2] Over 45 days beginning in January 2007, more than 17 million Hindu pilgrims took part in the Ardh Kumbh Mela at Prayag, and on January 15, the most auspicious day of the festival of Makar Sankranti, more than 5 million participated.[3]

The Maha Kumbh Mela ('Great' Kumbh Mela) which comes after 12 'Purna Kumbh Melas' which is after every 144 years is held at Allahabad [1][4][5]. The 2001, Maha Kumbh Mela was attended by around 60 million people, making it the largest gathering anywhere in the world

Amrut had to be saved from Danavas. So Gods had to hide it at different places - Swargalok, Mrutyulok and Patallok. Gods were on Mrutyulok (earth) for twelve days. Gods twelve days are equal to 12 years for Mrutyulok . Thus Gods were on earth for 12 years with Amrut. Hence every 12 years Kumbha Mela is celebrated at the four places, where a few drops of Amrut had fallen.

Otherwise people will not give it much importance