Dubbed as “America’s largest day of doing good, Make A Difference Day, is the fourth Saturday in October. ”

This year - Saturday, Oct 27, 2007

http://www.makeadifferenceday.com

There are all kinds of things you can join to help, or come up with something on your own.  You can even find local events featuring knitting, crochet, sewing, crafts, etc. if you enter the keyword on the Find a Project page.