The specific date for the full implementations / transition is not known yet as far as I am concerned. Vitalik Buterin, founder of Ethereum, announced late last year that he aimed to introduce what is called a Proof of Stake (POS) this year. The upgrade to the network is referred to as Casper. Instead of verifying transactions on the network via mining ( Proof of Work) , users will have the ability to ‘stake’ their Ether. If the stakers act truthfully, verifying legit transactions, they will certainly be compensated with interest / rewards proportional to their stake. Their stake will be shed if the stakers act maliciously and also attempt to cheat the system.
Although Proof of Stake is not fully implemented yet, On May 8th, 2018, Ethereum released the very first version of Casper Friendly Finality Device (FFG), which is a new POS algorithm on the Github coding repository. The release signified a massive progression towards the much awaited transition from existing Proof-of-Work (PoW) algorithm to Proof-of-Stake (PoS) algorithm.
It is also worth noting that Casper FFG is not meant to completely takeover the existing PoW consensus algorithm ( A complete shift towards full POS would probably takes year to happen ) but rather, contribute to a unique hybrid PoS/PoW consensus mdoel. The existing PoW algorithm will still be be utilized for generation of new block while the brand-new PoS will facilitate the process by providing an extra layer of security via a smart contract.