A few caveats: First of all, crowd counting of nonticketed events is a notoriously imprecise "science." Even the experts don't really know what they're doing. Anytime you see a crowd estimate, you should take it with a grain of salt. Second, defining "gathering of people" can be tricky because you have to set arbitrary limits on space and time. Do all the people have to be in one building? On one contiguous plot of land? In one city? Do they have to be there at the same moment in time, or can you count total attendance over the duration of something like an Olympics or a World's Fair?

We could say something like "30 million people visited Disney World during its 25th anniversary celebration" or, even more ridiculously, "one billion people gathered in China to ring in the new year." But I think that's getting a little too extreme.

So let's say that we're talking about a mass of people gathering, for a common purpose, at a place (a city or smaller) where they normally wouldn't be. And we'll set the time limit to one day. So this would include concerts, sporting events, political rallies, fairs, etc.

The "winner" is a surprise to me. It's none of the ones you mentioned, and it had only minimal television coverage (in North America, at least). It was the Maha Kumbha Mela in Prayag, India, on Jan. 24, 2001.

The Kumbha Mela (or Kumbh Mela) is a Hindu pilgrimage and festival that occurs every three years. The name "Kumbha Mela" means "urn fair"; it's based on the legend of a holy urn, holding the nectar of immortality, which was shattered in a battle of good and evil. Drops of nectar supposedly landed in four places; every three years, there is a pilgrimage to one of these places (Prayag, Harldwar, Ujjain and Nasik), and an accompanying festival. During the course of these festivals (they last more than a month), millions of people participate in religious ceremonies, doctrinal discussions, group singing, and — the main event — ritual bathing in a sacred river.

Once during every 12-year cycle, there is a Maha ("Great") Kumbha Mela at Prayag. This is considered the most sacred pilgrimage for Hindus, and tends to get the highest attendance. The 1977 event attracted 15 million pilgrims. In 1989, it increased to 30 million, with 15 million attending on the Amavasya day (the main bathing day). The Guinness Book of World Records acknowledged this as the "the largest number of human beings to ever assemble with a common purpose in the entire history of mankind." It appeared that the only thing that could top it would be another Maha Kumbha Mela. And that's what happened.

For the last Maha Kumbha Mela, in early 2001, 70 million attended over a 41-day period. On Jan. 24 alone (the Amavasya day), a reported 27 million people came to bathe. (I know it's for ritual purposes, but the first thing I'd want to do after bathing with 27 million other people is... go home and take a bath.)

Sources: KumbhaMela.net, Projectile Arts, A Description of the Maha Kumbha Mela Experience