We're about to migrate the servers running the Mediawiki application servers to a new release of the ICU Unicode library (https://en.wikipedia.org/wiki/International_Components_for_Unicode) (from 52 to 57). This unblocks some future work on upgrading the servers and will also allow the use of improved internalisation in the future (as wikis will then be able to use features introduced by the new release).
This migration will cause some unavoidable user-visible impact: The sorting of some category pages will be distorted; all pages which have been updated with the new software version
will use the new sorting while untouched pages still use the old sorting. As such, we need to run a maintenance script to update the sorting for old entries.
This affects a number of large wikis (including eight out of the ten biggest Wikipedias). The detailed list (and the task for the technical implementation) is at https://phabricator.wikimedia.org/T189295
The tentative date is the 9th of April 2018 (distortions may be visible for several days as the maintenance script takes quite a while to complete).
Please let me know if you need additional information.