When did Ethereum move from PoW to PoS?

When did the transition from Proof-of-Work (PoW) to Proof-of-Stake (PoS) occur for Ethereum, marking a significant milestone in the cryptocurrency's history? The shift has been long anticipated and has garnered immense attention from enthusiasts and industry observers alike. The move was heralded as a more energy-efficient alternative to the traditional PoW consensus mechanism, promising to reduce energy consumption by a staggering 99.95%. What specific date did this epochal transition occur, paving the way for Ethereum to enter the ETH 2.0 era and ushering in a new chapter for the blockchain network?