Maha Kumbha Mela also known as Purna Kumbha is held in Allahabad once in 12 years based on the zodiacal positions of Jupiter and Sun. Allahabad is the land where the three rivers Ganga, Yamuna and the mythical Saraswati merge into one another. Jupiter, the largest planet takes about 12 years for one revolution spending about a year in each zodiac sign. Hence, this mela is held once in 12 years when Jupiter is moving through Taurus.
Maha Kumbha Mela begins when Jupiter (Guru in Sanskrit) is in Taurus and the Sun is crossing over from Sagittarius to Capricorn to begin its northward journey. The holy dips in the mela continues for about 2 months under the watchful eyes of the Guru (Jupiter) while Sun moves through the zodiacal signs of Capricorn and Aquarius known to be ruled by Saturn. In Hindu mythology Sun is considered to be the father of Saturn (known to be cold and a hard task master). The most important bathing date in the entire duration of the mela is the new moon in Capricorn also known as Mauni Amavasya or Amrit Yoga when the water in the Ganga is believed to turn into nectar. New moons are also harbingers of new beginnings.
If you have missed the Maha Kumbh experience in 2013, get ready for the next one in 2025AD. 29th January 2025, will be the most important bathing day.