Google not set a date for the start of the algorithm Mobile First Index
He also added that preparations for the launch may take a few months more than previously thought.
Previously, that Google began testing mobile-first index in November 2016. This means that in the near future, search algorithms will be primarily to evaluate the relevance of the mobile version of the page and if its absence – the desktop.
It is noteworthy that mobile-first, Google will not measure the speed of loading the mobile pages and to take that factor into consideration in ranking. At least at first.
If the site may have problems after switching to a new type of indexing, Google will warn about it using the Search Console.
The FAQs on this change can be found here.