Google gave two months' notice, which was rare. Its post of 26th Feb 2015 said: "Starting April 21, we will be expanding our use of mobile-friendliness as a ranking signal."
In plain terms, pages that worked well on phones would rank better on phones.
The scope was broad. "This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results."
Google pointed site owners to its Mobile-Friendly Test and its Mobile Usability report to get ready. The same post made app content a ranking factor for signed-in users with the app installed.
Google did not use the name Mobilegeddon, and the change applied to mobile results only. It was the first step in a decade of mobile changes that ended with mobile-first indexing in 2023.
