Earth Hour is an annual international event created by the WWF (World Wide Fund for Nature/World Wildlife Fund), held on the last Saturday of March, that asks households and businesses to turn off their non-essential lights and electrical appliances for one hour to raise awareness towards the need to take action on climate change. It was pioneered by WWF Australia and the Sydney Morning Herald in 2007, and achieved worldwide participation in 2008.
Earth Hour will next take place on Saturday, March 28, 2009 at 8:30 pm, local time.*
More info: Earth Hour
*source: wikipedia – Earth Hour