Dates of the first Park(ing) Day are a little varied, but here's a little background info. This day is an opportunity to celebrate parks in urban cities and also to promote the need for more recreational and relaxing spaces. Living in a city like SF you tend to forget how important these public spaces are. Now that I'm lucky enough to live in a part of the city that's surrounded by parks (Duboce Park, Buena Vista Park, Dolores Park and Golden Gate Park), I've come to appreciate this movement.
Even if you don't plan on participating at least check out your nearest park. Folks have got some interesting ideas.