Gary Illyes at Pubcon announced today that Google is switching to a mobile first index.
This is a huge change. Google has always used the desktop version of a page for ranking, even when serving results to users on a mobile device.
Do note that this mobile first index is not live now. This is the pre-announcement so SEOs and site owners can be prepared for the upcoming change.
When Will Google Change to Mobile First Index?
Illyes did say that he will not give dates for this because dates can change. He did say that Google will be announcing it with a blog post, which will include details about when Google makes the change to mobile first indexing.
It is also worth nothing that Google has given plenty of notice about the changes they have made with mobile indexing, such as when they made the plan to add a mobile friendly ranking boost.