Checkout Kumbh Mela, India (https://en.wikipedia.org/wiki/Kumbh_Mela) going on right now. It happens every 6/12 years. In 2013, 120 million pilgrims attended it over period of 30-40 days.
With some metrics, I bet it still is. My understanding is (and please correct me if I'm wrong) that everyone is all going to a specific set of locations (instead of the more-broad "where family is"). What I'm not sure about is if everyone goes to those places in the same order, on the same days.