Churning of milky ocean was an uphill task. It was not possible to perform it without the help of Demons (Asurs). Hence Gods reached for an agreement with Demons to accomplish this task and it was decided that all perks coming out from the milky ocean would be shared equally including the nectar of immortality. During churning of milky ocean when the pot (Kumbh) of nectar came out, a fight broke out between the Gods and the Demons to get it completely without sharing it with the other.
According to Hindu mythology the fight between the Gods and the Demons continued for twelve years. During the fight the pot of nectar was shifted from place to place to keep it safe from demons. During this time few drops of nectar were spilled at twelve different places. Out of those twelve locations four were on the Earth and remaining eight were on the heaven.
Due to spilling of nectar drops, four places on the Earth, Allahabad, Nasik, Haridwar and Ujjain became sacrosanct. Hence Kumbh Mela is held only at these four locations.
Based on the astronomic positions of the Sun, the Moon and the Jupiter (Lord Brihaspati) the Kumbh gets repeated every twelve year. The Jupiter takes approximately twelve years to pass all 12 zodiacs and when it crosses Makar Rashi, which happens once in twelve year, there is Kumbh Mela at Allahabad. Jupiter remains in Makar Rashi for about one year and during this time one complete lunar month, during which Kumbh Sankranti occurs, is considered auspicious to take bath in sacred water of Sangam. Hence whole Magha lunar month becomes auspicious every twelve years to take holy dip in Sangam at Allahabad.
In 2013, due to transition of Jupiter through Makar, Kumbh Mela would be held at Allahabad during Magha month from 27th January to 25th February (both dates inclusive).