The Ubuntu 18.04 release date is set. For this, and other milestones in the Ubuntu 18.04 development cycle, check out the Ubuntu 18.04 release schedule.
The Ubuntu 17.10 release date is set for October 19, 2017. Ubuntu 17.10 will use the GNOME desktop instead of Unity, the first release to do so since 2011.
GNOME 3.24 is to be released on March 22, 2017 — providing that development goes to plan between now and then, of course!