Why is Kumbh festival celebrated in India?
Kumbh Mela is one of the biggest festivals in India in which people and devotees from all over the world gather.
In which cities Kumbh Mela is celebrated: –

Kumbh festival is celebrated after every 12 years in India on the banks of the river in the following place, where Yamuna and Saraswati meet.
- Ganga in Haridwar
- Shipra in Ujjain
- Godavari in Nashik
- Triveni in Prayag
Samudra Manthan :-

According to mythological beliefs, it is believed that when the sea was being churned, when the poison came out in that churn, nectar was also received. Mahadev kept the poison in his neck for the welfare of the universe, while the nectar on the other side Many people came forward to get.
Amrit between God and demons :-

Taking the nectar, there was a clash between Dev and the Danavas and both turned against each other, the gods wanted only the nectar to be given to them and the demons wanted the nectar to be given to them only so that they would become immortal and for centuries the whole creation was deviant. To spread the power, some drops of nectar fell on the earth between these clashes, this is the place where the Kumbh Mela is held, that place is Haridwar, Ujjain, Nashik, Prayag.
Why does kumbh happen after 12 years :-

It is believed that there was a continuous battle for the nectar for 12 days between the gods and the Danavos, it is also believed that 1 day of the gods is equal to 1 year for the earth dwellers. Thus the war of the gods and Danavos for 12 years This is why the place where the nectar fell is considered worthy of the Kumbh Mela and the Kumbh festival is celebrated every 12 years.