Web developers who haven't yet optimized their Websites and Web apps for mobile devices may want to do so before April 21. That's the day when Google will begin taking a site's mobile-friendly characteristics into account in its search results. As a result, some are calling the day "mobilegeddon."
It isn't exactly clear what characteristics the Google algorithm considers when determining how mobile-friendly a site is. Many developers are using responsive design to create a mobile version of their sites. Experts also recommend making sure that Google's bots can crawl your mobile site and checking every page from a mobile device to make sure the navigation works as intended.