Ethereum Metropolis - 25th June - mentioned by Vitalik Buterin in Core Dev Meeting

METROPOLIS is the next Ethereum Hard Fork and the third major network release.

Ethereum Core Devs Meeting #14 [4/21/17]

https://github.com/ethereum/pm/blob/master/All%20Core%20Devs%20Meetings/Meeting%2014.md

@ 38:22 in the Core Devs Meeting Vitalik mentioned a 25th June Metropolis release, basically if the hashpower stays the same as it is now.

But it is more likely that the hashpower will continue to increase till this date, which would probably delay the Metropolis Hard Fork for a couple of weeks.
So maybe to expect till the end of July
This has to do with the correlation of Hashpower and the affected Block time.
The maximum acceptable Blocktime represents 30 seconds for the Core Developers.

Metropolis

Metropolis is the release that opens the gates to the masses. This is where there are fully fledged and tested user interfaces for the non technical users. Although this is now being performed by individuals - for example the ethereum desktop wallet. Again it is unclear when this phase will take place and whether it depends on the pace of external community development.

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