From the official site:
On March 29, 2008 at 8 p.m., join millions of people around the world in making a statement about climate change by turning off your lights for Earth Hour, an event created by the World Wildlife Fund.
....We invite everyone throughout North America and around the world to turn off the lights for an hour starting at 8 p.m. (your own local time)–whether at home or at work, with friends and family or solo, in a big city or a small town.
If you haven't already heard of this, it's a bit last minute on the notice, I know, but hey! you can always do it tomorrow. Every light counts.