How do I add German public holidays to my Outlook 2016 OSX App
It’s true, in Mac Office 2016 there’s no longer an option to add holidays. Also, you can’t download a *.ics calendar file and import it into Mac Office 2016. Both workaround only apply to Mac Office 2011, only.

The only way around this is to add your Exchange/CalDAV account to Apple Calendar App. Than import your German-Holiday.ics file to your Exchange account. Finally open Outlook and wait until all calendar items have been synced.

UPDATE Juni 2019: I am getting back to my original answer from 2 years ago. In the meantime Microsoft silently added a couple feature to open ics files. Outlook still lacks a import feature in the Import... menu. But user Rick Troiani pointed out a really nice workaround. 

Solution A: Drag-and-drop your *.ics file right onto my Outlook Calendar. 

...You can also use the following workflow:

Solution B: Drag-and-drop your *.ics file onto the Outlook icon within the macOS Dock. 

Solution C: Right click your *.ics file in macOS Finder > Open with Outlook.