On the last Saturday of March, every year, people around the world turn off their lights, computers, electronics, and televisions to help save the Earth. You can read more about Earth Hour at EarthHour.org
At a time you choose, your site can be inaccessible and display a notice with information about Earth Hour. The app runs on browser time, so best practice is to set it and leave it. It will not activate until next year's Earth Hour.