The new version of MediaWiki will be on test wikis and MediaWiki.org from September 1. It will be on non-Wikipedia wikis from September 2. It will be on all Wikipedias from September 3 (calendar).
You can join the next meeting with the VisualEditor team. During the meeting, you can tell developers which bugs are the most important. The meeting will be on September 1 at 19:00 (UTC). See how to join.
The Wikidata API will be updated with breaking changes. This will probably happen on September 9.