We are heading towards the Isabelle2018 release, which is scheduled for August 2018. This spot provides a reference for upcoming release candidates.
The Isabelle release process is subject to the laws of causality: release candidates can be modified, but the final release is unchangeable. This means that testing needs to happen in the weeks before the final release.
The main forum for discussion of Isabelle2018 release candidates is on isabelle-users mailing list.