Google's Search Liaison account confirmed a broad core algorithm update on 20th Apr 2018. The rollout had begun the previous Monday, 16th Apr, as reported by Search Engine Land.
It followed the broad core update Google had confirmed in March, a few weeks earlier.
Google's wording 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. No formal end date for the rollout was announced.
For sites that moved in mid-April 2018, the standing core-update advice applies. Review the affected pages against the self-assessment questions in Google's core updates help pages.
Invest in overall quality. Recovery typically shows up at a later core update.
Our recovery guide covers the sequence.