On Fridays get yourself to Midan Talaat Harb in Cairo at 9am. It's early on the first day of the weekend (Islamic Sabbath) so traffic will be light. In the square (perhaps on the large section of sidewalk in front of the Tulip Hotel) you'll find the Cairo (Sahara) Cycling Coalition.

At least this was my experience on May 22, 2009. They're still working out an "official" critical mass plan, but hey, last Friday of the month and you're on a bicycle, how bad could it be?

If you're itching to ride on your own, I'd recommend Luxor.