The biggest dedicated Attack on Titan fan event is probably Fall of Wall, the fourth iteration of which is scheduled for October 20th. However this one probably has the best location. The venue for the September 29th cosplay gathering is a dam rather than a wall, but wait until you see the Oyama Dam, near Attack on Titan author Hajime Isayama's home town of Ōyama, Hita, Ōita Prefecture. It'll certainly look familiar.
The cosplay event will be 10:00 am until 5:00 pm. A 1,500yen entrance free includes snacks, accessing to parking, changing rooms and photography spots by the dam, lawn and park.