Mobile-friendly update (Mobilegeddon)
Mobile-friendliness became a ranking signal for mobile searches, announced almost two months ahead of launch.
What changed
Google gave unusual advance notice that from 21 April 2015 mobile-friendliness would be used as a ranking signal on mobile searches, evaluated page by page and applied in real time. It provided a mobile-friendly test tool and a Search Console mobile usability report so publishers could prepare. The measured impact was smaller than the pre-launch coverage suggested, but the announcement drove a wave of responsive redesigns. A strengthened second version followed in May 2016.
Timing
Announced on 26 February 2015, with rollout beginning 21 April 2015. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.