The release of Isabelle2015 is scheduled for this spring, presumably the end of May, or the start of June. As usual, there are several weeks for thorough testing of release candidates that are available here:
With ever increasing size and complexity of the system and its many add-on tools, it is important to take testing of release candidates seriously. Problems can be solved before the release, not after it!
The main forum for discussion is on isabelle-users mailing list.
The convergence towards the final release happens on the following repository.
Update: The final release of Isabelle2015 (May 2015) is now available from the Isabelle website. The above release candidates will disappear eventually.