Today, the pretesting has begun for the Ethereum Fork. Here is a quote from Coindesk:
"Byzantium, the first part of ethereum's long-awaited Metropolis upgrade, has officially launched on testnet.
The simulated hard fork was executed this morning on Ropsten, the ethereum testing environment, and is expected to run for a few weeks of troubleshooting before the fork occurs on the main ethereum blockchain, currently the world's second-largest by market capitalization."
Testing will occur for about 3 weeks and then the developers will announce an exact date when the hard fork will occur:
"The ethereum developers are expected to announce a formal date for the hard fork shortly – provided everything runs according to plan."
Sources:
https://www.coindesk.com/ethereums-next-hard-fork-now-officially-tested/
http://status.ropsten.ethereum.org/