If your website is not mobile-friendly by April 21st, Google will throw your website under the bus.
I have seen it before. Now that 60%+ of website views are on mobile devices, Google has decided to punish websites that don't work on them.
When the Panda and Penguin algorithm happened, five websites that I was manager of SEO for went from #1 on Page 1 on Google to page 8, 10, and worse. The new mobile-friendly algorithm has been dubbed "Mobilegeddon" by the Entrepreneur
Google's new ranking system will be run in real time and will works on a page-by-page basis. When the Google spider crawls through all of the websites over the week after April 21st, it will immediately rank the website as Mobile-Friendly, or not Mobile-Friendly.
If it is not ranked as mobile-friendly, you need to get on schedule for a website upgrade ASAP. Failure to get your website mobile-friendly before April 21st will result in a rank decline that may take months to regain, even if the website is made mobile-friendly quickly afterwards. Like I said, I have seen this happen before when the Penguin and Panda update came out, and this has been described as more catastrophic by bloggers. Google said that the change will have a "significant impact" on search results across the globe.
In addition to the mobile-friendly algorithm, Google is also including an app indexing factor that requires app developers to add deep link support, verify the apps website on Google Play, provide deep links for each page, and ensure that there are no errors in webmaster tools for the app.
Go here and verify that your website is mobile-friendly: https://www.google.com/webmasters/tools/mobile-friendly/
If it's not, call me right now at (707) 293-7821.