Estonia "Let's do it!" on 1st of May (Tallinn)
There's at least one English-language group as well, and definitely want to be there!
"Want to make a good country better? A little "remont" where you don't have to get your hands dirty? Now's your chance. The whizkids behind Estonia's legendary countryside cleanup (50,000 volunteers, 10,000 tons of crap, one single day) have put together what may be the world's largest brainstorming event. Over 100,000 Estonians will come together on May 1st to discuss how to make the country better--and they want to hear from everyone. This means you."