The announcement will come in April at the Microsoft BUILD 2014 conference in San Francisco. In the announcement Microsoft will announce that Windows 9 is planned for a April 2015 release.
According to what has been able to be uncovered about Threshold, rumors suggest that it will see not only the return of the “Start” button as a stock part of the build, but it will have the ability to run both regular desktop applications side by side with the Metro style applications. If accurate, it would be the version of Windows that both home and business users have been asking for and the reason that Windows 8 was such a flop.
See Thurott’s confirmation of the Windows 9 release date here.