Here's why the group is doing this (according to there website):
To tackle climate change we need to move quickly, and we need to act in unison—and 2009 will be an absolutely crucial year. This December, world leaders will meet in Copenhagen, Denmark to craft a new global treaty on cutting emissions. The problem is, the treaty currently on the table doesn't meet the severity of the climate crisis—it doesn't pass the 350 test.
In order to unite the public, media, and our political leaders behind the 350 goal, we're harnessing the power of the internet to coordinate a planetary day of action on October 24, 2009. We hope to have actions at hundreds of iconic places around the world - from the Taj Mahal to the Great Barrier Reef to your community - and clear message to world leaders: the solutions to climate change must be equitable, they must be grounded in science, and they must meet the scale of the crisis.
As I write this blog post, there are more actions being formed. At the moment there are 3257 actions being planned over 159 countries, and there are 6 days left! If you want to find the action closest to you, create your own, or find out more about the movement – please visit http://www.350.org/.