Phase zero is the name of the first transition stage of the Ethereum network from a proof-of-work to a proof-of-stake consensus algorithm, which would pass block validation function from miners to special network validators.
“We still have quite a bit of time before the end of 2019, so I think looking at a target genesis date towards the end of 2019 could be realistic.”
Drake also considered the issue of deposit contracts, suggesting to launch those ahead of the targeted genesis block to allow time for validators to make deposits. The developer said that the deposit contract ceremony will be held at Devcon, a Ethereum developers conference in October. According to Drake, such timing will enable developers to deliberate on the address of the deposit contract, as well as to avoid scam deposit contracts.