Blizzard will be performing maintenance on US West on January 17, 2017 from 10:00 AM - 1:00 PM PT (UTC-8).

Official Announcement: https://us.battle.net/forums/en/bnet/topic/20752529363

Update: Blizzard is pushing the maintenance to January 19, 2017 from 7:00 AM to 12:00 PM PT.

Source: https://us.battle.net/forums/en/bnet/topic/20752529363#post-5

Edit by Carl Bennett: This maintenance appears to have enabled the ability to register illys (accounts with names that contain what used to be invalid characters).