This Washington Post article has all of the details:

The [GovLoop] Web site’s creators are buying lunch for the first 500 public servants who show up on Dec. 8 from 11:30 a.m. to 1:30 p.m. at the L’Enfant and Federal Triangle metro stops. Federal, state and local government employees who show up with a government-issued ID or business card will be able to choose between free barbeque [sic] or macaroni and cheese lunches courtesy of the BBQ Bus or Cap Mac food trucks.