SharePoint Server 2016 IT Preview has already been released in August 2015.
(For download click here)
Now the release date of the SharePoint Server 2016 Beta 2 Version has been announced here in Stockholm at the European SharePoint Conference 2015.
The Beta 2 version will be available still in November 2015, propably by Thanksgiving 2015.
99% of the final release features will be available in the beta release.
There is also a commitment of Microsoft, that they will support and release additional Server versions after the 2016 release.