Which Hindu festival celebrated every 12 years is the largest in the world?

It's a festival in Hinduism called Kumbh Mela. It's celebrated in a roughly 12-yearly cycle that coincides with the planet Jupiter's orbit.

There are four pilgrimage sites: river banks of the Ganges/Yamuna/Sarasvati rivers' confluence, Ganges, Godavari and Shipra rivers.

The festival is marked by a ritual dip in the waters, but it is also a celebration of community commerce with numerous fairs, education, religious discourses by saints, mass gatherings of monks, and entertainment.

For believers, bathing leads to atonement and penance for past sins which are cleansed.

https://en.wikipedia.org/wiki/Kumbh_Mela

Tens of millions of men, women and children participate in this festival.
http://www.pbs.org/wgbh/sacredjourneys/content/kumbh-mela/