Originally, AMD planned to launch the RX Vega 56 in September with reviews coinciding with that launch date. However, AMD decided that an earlier launch of the RX Vega 56 would be more beneficial. Presumably, these cards will be more competitive versus NVIDIA's offerings than the Radeon RX Vega 64.
The end result is that RX Vega 56 reviews will be published on August 14th, with initial availability expected on August 28th.