"On Independence Day, to thank America’s troops for their sacrifice and service, active military personnel and veterans will be admitted with one guest to see Columbia Pictures’ White House Down for free at any participating Regal Entertainment Group, AMC Theatres, Cinemark Theatres, Carmike Cinemas theater or other participating theater by showing proper military identification. Please check your local theater for participation. The special invitation to America’s military personnel and veterans is valid only on the 4th of July and seating is available on a first-come, first-served basis at each participating location."
This was posted on their Facebook Wall here. I suggest you call your movie theatre before you go to make sure they are aware of the promo so you don't have any problems.