What’s better than mimosas with brunch, walks in the park, dinner and a movie, day tripping out of town, reading a good book, and any other fabulous thing you could possibly think to do on a Saturday? Saving humanity from itself, that’s what.

Saturday, October 24, has been dubbed International Day of Climate Action by activist/author Bill McKibben, along with a phalanx of high-profile enviro groups (including Friends of the Earth, Sierra Club, and Greenpeace), and citizen treehuggers like yourself. So far, 4043 “actions” – rallies, art expos, walks, bike rides, sing-alongs, interfaith prayer meetings, trash clean ups, sustainable cook-offs, and other events designed to draw attention in creative ways to climate change – have been slated to take place in 168 countries from Denmark to the UAE. And with just four days left to blast off, the campaign seems to be seeing a last minute rush; the number of events rose by more than 100 in just the time it took to write this blog.

So what’s the big deal about 350? It’s nature’s carbon cap. It’s the greatest amount of carbon dioxide (measured in parts per million, or ppm) that our planet can sustain in the atmosphere while still supporting life as we know it. The world is already experiencing the ramifications of climate change, but anything beyond 350 ppm is where things get really messy.

NASA scientist Dr. James Hansen explains is this way: “If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggests that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.”

But hang on, you’re saying. If we’re already at 385ppm now, and anything above 350ppm spells doom, shouldn’t we just throw our hands up, leave all the lights on all the time, turn the heat up to 84 degrees, buy the cheapest instead of the greenest products at the grocery store, and give up on making renewable energy competitive with oil and coal? No, say the campaign’s organizers: “We’re like the patient that goes to the doctor and learns he’s overweight, or his cholesterol is too high. He doesn’t die immediately – but unless he changes his lifestyle and goes back down to the safe zone, he’s at more risk for heart attack and stroke. The planet is in a danger zone because we’ve poured too much carbon into the atmosphere, and we’re starting to see signs of real trouble: melting ice caps, rapidly spreading drought. We need to scramble back as quickly as we can to safety.”

So click here to find an action near you, to organize your own, or to check out suggested action ideas. But try to come up with something unique before you look to the campaign’s suggestions. One woman in NYC has posted on the site to say she’s going to sit by the Hudson River and sing a dirge for life on planet earth, if that gives you a sense of where the limit is. (The Sky.) And don’t forget your camera on the big day; 350.org wants everyone to post photos of their Oct 24 adventures to the site.

Big names aboard the 350 train include: IPCC chairman Rajendra Pachauri, Canadian Inuit activist Sheila Watt-Cloutier, Worldchanging exec editor Alex Steffen, polar explorer Will Steger, NASA’s Dr. James Hansen, “No Impact Man” Colin Beavan, Green Collar Jobs proponent Van Jones, Archbishop Desmond Tutu, and Maldives President Mohamed Nasheed.