Google confirmed on March 12, 2018, via its Search Liaison Twitter account, that a broad core algorithm update had been released the previous week, as reported by Search Engine Land. Sites had already been reporting ranking fluctuations for days, and the update is dated to March 9, 2018, when that volatility spiked. No formal end date was announced.
The confirmation set out language Google would reuse for years: it does these updates "routinely several times per year", pages that drop are not broken, and "changes to our systems are benefiting pages that were previously under-rewarded". Google added that there is "no 'fix' for pages that may perform less well other than to remain focused on building great content". The same framing reappeared in the April and August 2018 confirmations.
If a site moved in this window, the guidance that applies to every broad core update applies here: assess the pages that lost visibility against the self-assessment questions in Google's core updates documentation and improve overall quality, with recovery typically registering at a later core update. Our recovery guide covers the sequence.