At 8:30pm local time, I'd love for whole parts of the world to get dark, but I doubt that will happen, so it'd just be cool if whole areas went out for it. It's meant to be a call for action on climate change, but you can read more about at their site. I think it's really just very good idea for the lights to go out at night so that people can look up at the stars and planets. There's a lot of light pollution, and it's rather boring to look up at a sky with very few stars. I'll probably spend the hour reading by flashlight or something like that.
I hope that this is rather new and that it will gain more notoriety as the years continue. I'm going to add it to my calendar.