GNOME 3.18 is expected to released next September, but until now we just have some information about the direction of 3.18 branch. GNOME 3.18 will focusing on Files and Calendar development.
Without a doubt, Files and Calendar are among the most important packages in the GNOME stack. Any changes made to these two components will be immediately spotted by users, so the developers need to be very careful what modifications they make. Now that we can see what improvements are going to land GNOME, well ahead of time, this is a good way of preparing the users for what’s coming.
GNOME 3.18 should be released on September 23.