The beginning of the system-wide transition known as the Merge will be on September 6th, according to the foundation. The first domino will fall on the 6th and the rest of the process will begin on the 10th.
The switch over from proof-of-work to proof-of-stake is referred to as the "Merge". The technical differences between the two approaches are a little arcane, if you aren't interested in the world of crypt. The new proof-of-stake method will use less energy than before. This is a big deal because of the widespread criticisms of Cryptocurrencies as a waste of power.
The Merge is a two-step process — September 6th is just the first step
The foundation describes the process as a two step process. The upgrade will go live on September 6th at 11:30 AM. When this is live, it starts another counter towards a terminal total difficulty of 587500000000000. TTD is a measure of activity on the ether network. Once this threshold has been reached, Paris will be triggered and the Merge will be completed. The tangible effects of the switchover will take a long time to play out.
The Foundation can't give an exact date for the switchover as it depends on the amount of ether used after September 6th. They think it will be reached by September 15th.
One of the biggest events in the world to date is the merge. It is a technological upgrade that will allow new features, but it is also a test of the community's commitment. It will be a sign of confidence in the platform if it thrives after the merge.