The much-anticipated Ethereum hard fork known as Constantinople is set to be implemented at block number 7,280,000, which developers have predicted will take place on Thursday, February 28. Of course, the exact timing of activation depends on block times between now and then, meaning the upgrade could happen one to two days before or after this Thursday.
Constantinople was initially set to activate last month, but a security vulnerability in EIP 1283, one of the originally included improvement proposals, delayed the upgrade's implementation. Core developers decided to remove the faulty EIP and push the hard fork back to block number 7,280,000.
To remove EIP 1283 from testnets like Ropsten, which had implemented the change prior to the vulnerability's discovery, a second upgrade called St. Petersburg will occur at the same time as Constantinople. Moreover, despite increased discussion regarding the CREATE2 opcode and its reported security vulnerability – which would enable attackers to exploit "self-destructed" contracts by deploying different code to the same address – that proposal (EIP 1014) will still be included in Constantinople.
Besides the removal of EIP 1283, Constantinople remains the same; "average" Ethereum users and Ether holders do not need to do anything to prepare for the upgrade. However, node operators and miners need to download the latest version of whichever Ethereum client they run to ensure they can participate within the post-upgrade Ethereum network. Not upgrading will leave node operators and miners on an incompatible chain and, thus, prevent them from receiving and sending ETH.