Purna Kumbh was held within 11 years because of the favorable astrological position of the "Sun moving into Aries and Jupiter into Aquarius" in 2021.
Kumbh Mela is a Hindu religious festival that was held in Haridwar, Uttarakhand, in 2021. The Purna(Full) Kumbh should have been organized in 2022 but was scheduled a year earlier due to certain astrological positions. The rationale being the Sun entering Aries and the Jupiter entering Aquarius in 2021 instead of 2022, making this mega event "unique and auspicious," according to Haridwar Kumbh Mela's official website.
Every 12 years, the Kumbh Mela is organized in one of the four pilgrimage sites located on the banks of the holy rivers - Haridwar on the Ganges in Uttarakhand, Nashik on the Godavari river in Maharashtra, Ujjain on the Shipra river in Madhya Pradesh, and Prayagraj at the confluence of three rivers Ganga, Yamuna and Saraswati in Uttar Pradesh. The astrological calculation rules where the Kumbh takes place. The last Purna Kumbh was also organized in Haridwar in 2010. The dates of the Maha Kumbh Mela are determined following methods mainly with the "planetary positions(Zodiac signs)."
There are three types of Kumbh, and the regular Kumbh is held every three years, Ardh(Half) Kumbh in six years, and the Purna Kumbh in twelve years. It takes 11 years and ten months(approx 12 years) for Jupiter to complete the twelve zodiac signs cycle as each planet takes a different time to complete one revolution around the Sun. This completion results in Purna Kumbh.
In January 2019, it was decided that the Kumbh would be held in 2021 as determined by the Ganga Sabha, a group of priests responsible for the maintenance of Har-ki-Pauri at Haridwar, The Times of India reported.
Hence the Kumbh Mela was organized in 2021 instead of 2022, factoring in the astrological significance.