Geo Cruise, the Big Apple Tweed Ride (in NYC) just happened this past Sunday. But as noted above, they have two per year.
We had our first tweed ride here in NJ this past May. I was the organizer. We held it in South Orange. Lots of pictures
here. I'm the guy in the middle on the red bike. It's my 1967 Hercules 3-speed, made by Raleigh. As you can see, we tried each other's bikes. We beat NYC, having our first before theirs.

The weather was warmish, so I got pretty hot. I don't find tweed to be very breathable! It's woven -- not knit -- very tightly. It is quite durable, however.
Look at the pictures of the Portland, OR ride. Looks great, with a huge turnout.
Yes, this all started in London in January, 2009. It's been spreading like wildfire. One woman walking on the sidewalk in South Orange saw us and laughed loudly.