Some things will not work for about a minute on 5 May. This will happen around 06:00 UTC. This will affect the content translation tool and notifications among other things. This is because of an upgrade to avoid crashes.
Changes later this week
Reference Previews will become a default feature on a number of wikis on 5 May. This is later than planned because of some changes. You can use it without using Page Previews if you want to. The earlier plan was to have the preference to use both or none.
The new version of MediaWiki will be on test wikis and MediaWiki.org from 4 May. It will be on non-Wikipedia wikis and some Wikipedias from 5 May. It will be on all wikis from 6 May (calendar).
The CSS classes .error, .warning and .success do not work for mobile readers if they have not been specifically defined on your wiki. From June they will not work for desktop readers. This can affect gadgets and templates. The classes can be defined in MediaWiki:Common.css or template styles instead.