Info from secularstudents.org (use the search bar in upper left corner of home page):
National Ask An Atheist Day (Third Thursdays in April) is an opportunity for secular groups across the atheist country to work together to defeat stereotypes about atheism and encourage courteous dialogue between believers and nonbelievers alike. The event is intended to be an opportunity for the general public - particularly people of faith - to approach us and ask questions about secular life. It is our hope that encouraging participation on a national level will raise awareness of the event; we're encouraging all SSA affiliate groups to participate at whatever level they are able!
Story in Hemant Mehta's Friendly Atheist blog