Google began testing the mobile-first index in November 2016, though talk of it first began in September 2015. They have chosen this path since the majority of their traffic comes from mobile devices. We’ve heard that the launch could be happening soon, and Google has hinted that your mobile site needs to be responsive ahead of the launch to make sure your desktop and mobile pages are equivalent to one another.
The Mobile-First Index doesn’t have a set launch date, and it looks as though the rollout will be gradual, and beginning sometime in early 2018. Because we could be wrong since Google likes to surprise us, it’s better to get ready now than to be caught scrambling when announcements are made. It’s not likely the rollout will happen all at once. It’s expected to be completed on a site-by-site basis, rolling the index out in batches.