Segregated Witness (BIP141) activates on Bitcoin mainnet at block 481,824 on August 24, 2017. The upgrade fixes transaction malleability, enables the Lightning Network, and effectively increases block capacity to approximately 2-4 MB through witness data discounting. The culmination of years of development and the blocksize war, SegWit's activation was a victory for backward-compatible soft forks and user-driven governance. The first SegWit transaction was included in the activation block itself, and adoption would grow steadily to over 80% of transactions in subsequent years.
BIP 141: Segregated Witness