Google's Search Liaison account confirmed on April 20, 2018 that a broad core algorithm update had begun rolling out the previous Monday, April 16, as reported by Search Engine Land. It followed the broad core update Google had confirmed in March, a few weeks earlier.
Google's framing was notable and has been quoted ever since: "There's nothing wrong with pages that may now perform less well. Instead, it's that changes to our systems are benefiting pages that were previously under-rewarded." The company added that there is "no 'fix' for pages that may perform less well, other than to remain focused on building great content." Google did not name a target, and no formal end date for the rollout was announced.
For sites that moved in mid-April 2018, the standing core-update playbook applies: review the affected pages against the self-assessment questions in Google's core updates documentation and invest in overall quality, with recovery typically registering at a later core update. Our recovery guide covers the sequence.