This year, Clean up The World Weekend is from 19th - 21st September.
Clean Up the World partners with the United Nations Environment Programme (UNEP) to inspire and assist communities to "clean up, fix up and conserve their local environment" through carrying out initiatives ranging from waste removal and tree planting to water and energy conservation projects. (Information obtained here)
Although there's a certain period for this purpose, it does not mean we don't clean up other days! Everyday should be a Clean Up Day, Earth Day, World Environment Day! ;-)