Earth Hour started in 2007 in Sydney, Australia when 2.2 million homes and businesses turned their lights off for one hour to make their stand against climate change. In March 2009, hundreds of millions of people in over 4000 cities in 88 countries switched off their lights to pledge their support for the planet, making Earth Hour 2009 the world’s largest global climate change initiative.
Here's the link for more information.
Earth Hour 2010 takes place on Saturday 27 March at 8.30pm (local time) and is a global call to action to every individual, every business and every community throughout the world. It is a call to stand up, to take responsibility, to get involved and lead the way towards a sustainable future.