First of all, sorry for assigning this directly to @CKoerner_WMF - I just did it because you kindly helped us with (T176883) so I thought it would be a good starting point. But please, feel free to reassign it to your convenience or point me in the right direction.
We are ready to split dewiki and wikidata into a separate set of servers (T177208).
In order to do that, we would need a 30 minutes period of read-only time on s5 (that means dewiki and wikidata) so we can do the split.
Date: 9th January 2018
Time: 06:00 UTC - 06:30 UTC
User impact: During this period writes will not be allowed. That means edits will not be possible on dewiki and on wikidata
Reads will continue as normal. They will not be impacted. So everyone will be able to browse those two wikis without any issues.
Can you help us communicating this?
Thanks again and sorry for assigning this directly to you.