For a more complete calendar listing events for many secular organizations, see the Secular Coalition for America's Calendar page.
From the Reason Rally website:
In 2012, people from all across America assembled together on the National Mall in D.C. for an epic event in the history of our country. Standing in the rain, we declared to the world, “We are many! We are strong! We are united! And we are here!” Four years have past and we’re ready to do it again!