The Ethereum Foundation recently confirmed that the the mainnet chain will merge with the beacon chain sometime between September 10 and 20, depending on fluctuations in the network's hash rate, which itself is a measure of computing power.
However, as Ethereum co-founder Vitalik Buterin reminded his Twitter followers on August 29, a crucial step in the process—the beacon chain hard fork—is scheduled to take place on September 6. So node operators are expected to upgrade their clients before this happens.