It will take place at 8:30 PM Saturday your local time. More information at this link: http://www.earthhour.org/
And yes, turning the lights off for an hour is a very small thing. But Earth Hour has become a worldwide phenomena which helps raise awareness of this more than vital issue. It has grown from one city to 152 countries. Earth Hour also now features other actions to support the planet. Those include the Earth Hour City Challenge which "has been created to mobilize action and support from cities in the global transition towards a climate friendly one-planet future."
Go dark to keep the green!