A few things get a webmaster’s attention, like waking up to a Twitter feed bombarded with news about a major algorithm update. Most algorithm updates come with vague or no warning, but Google recently announced “starting April 21, we will be expanding out use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have significant impact in our search results”.
Google is providing a timeline to encourage those who haven’t already adapted their websites to mobile to begin making this transition which means that you should work towards having a mobile-friendly website by April 21 because it’s apparent that Google intends to reward mobile-friendly websites with better rankings in mobile search. Websites that are not mobile-friendly may very well experience a dramatic decline in mobile rankings.