Microsoft is currently aiming to hold a press event to introduce Windows 9, currently known as Windows Threshold, on September 30th. This date may change, but a Technology Preview of the operating system will be distributed to developers and enterprise users in late September or early October. The early Technology Preview should provide a first look at the new mini Start Menu, the removal of the Charms bar feature, and several UI changes to the way the desktop operates with Windows 8-style applications. Future versions are expected to include a version of Cortana, alongside other new features that Microsoft is working to complete ready for release in spring next year.
Update 7AM ET: article updated to confirm the Windows 9 logo that Microsoft used is simply a mockup.