Kumbh Mela

This is a religious and cultural celebration, but it only happens four times every 12 years!

It is a huge pilgrimage that goes to the Ganges river, but they can relocate depending on the position of Jupiter.  At the most, 60 million people come to celebrate every four years.
By the way, a Pilgrimage is an especially long journey to a sacred place to pay homage.
 Easy enough, eh?  :) :) :) :) :) :) :) :) :) :) :) :) :) :) :)