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In Portland it's the Bridge Pedal. Riders who go on a variety of routes that cross some or (almost) all of Portland's bridges. The best part is that they shut down I-405 and part of I-5. Ridership has been down over the past few years, though. I think that it peaked at around 20k a few years ago when it was truly done across all bridges, but last year was less than 10k.
There's also the World Naked Bike Ride but that always winds up being such a clusterfuck that people just stand/walk next to their bikes for most of it. Not what I'd call a cycling event. And the smell is horrifying.