I'll turn the floor over to Amma. "It's sort of a white people's Carnival. And since it's cold, they can make more creative costumes because they don't have to be concerned with, you know, battling the heat with nakedness." And to continue the parallel, the fancies are sort of like Carnival bands, kinda sorta not really, only that Mummers tend to be older and fatter as well as paler than your typical Carnival reveler. Which makes it a good thing that they wear more clothes.
Boring serious grown-up types have asked me to put the rest of the revelry below the fold.
So among the Star Wars junkies, you also had political messages:
Cavalry dancing with Indians:
And things that made no goddamned sense at all.
I don't know if they suspended the alcohol laws or just didn't bother to enforce them, but never in North America have I seen so many people wandering around with open bottles and cans of beer but nary a paper bag in sight. By nightfall, some streets looked like it had snowed colored aluminum.
Supposedly there's a long history here, dating back to the 1840s, combining ancient Irish and Swedish and Finnish customs, whatever, yadda yadda. (The main newspaper also warned people not to fire their guns into the air at midnight, and then pinned the custom on Finland. Jussi?) I'll let those who know more than you can find out in two minutes of googling explain it all in comments.
All I can say is that it's hella fun, and worth spending your next New Year's in Philly to participate in. And I mean "participate": it's so unprofessional, and starts so far south along Broad Street, that you can't not get involved in the action. So next year, in 2009, see y'all in the City of Brotherly Whatever the Hell it Is.