Here's a link from the Happy Atheist on Twitter: http://reasonrally.org/

"The Reason Rally is an event sponsored by many of the country’s largest and most influential secular organizations. It will be free to attend and will take place in Washington, D.C. on March 24^th, 2012 from 10:00AM – 4:00PM at the National Mall. There will be music, comedy, speakers, and so much more. We hope you can join us!  Please poke around this site for more information..."