Critical Mass is a cycling event typically held on the last Friday (or in Vienna: 3rd Friday) of every month in over 300 cities around the world. The purpose of Critical Mass is to reclaim the streets for bicycles, skaters and other forms of transportation aside from cars. In Vienna, the Critical Mass is continuously held every month since march 2006 and has had events with serveral thousand people.
To learn more about CM visit www.criticalmass.at (german) or Wikipedia https://en.wikipedia.org/wiki/Critical_Mass