Byzantium is the first of the two phases in the Metropolis update, which is to be followed by Constantinople later.
The date of a launch for the Byzantium testnet, the next critical step towards Metropolis, a major upgrade for Ethereum, has been announced.
Thus, the testnet will be deployed on September 18th.
According to the information provided by coindesk.com, the actual network upgrade release is to be launched in October, depending on the length of the testing period and any issues that may arise during that time.
If things go wrong....they will go wrong fast.
Vitalik Buterin, the creator of Ethereum, argues that he anticipates the Byzantium test phase will last between three and four weeks.