SOPlanning comes with a feature to import holidays to your planning.
There is a nice set of ics files to choose in the "Settings, Public holidays" menu.
Clicking on the "Import by country" button opens a dropdown list.
It shows the files which are located in ../soplanning/holidays.
However, the German file "GermanHolidays.ics" contains some days which are no public holidays and it misses some other public holidays that do exist.
A more complete alternative can be found here:
https://www.feiertage-deutschland.de/kalender-download/
Go to the website, download the ics file "Feiertage", rename it to "AllGermanHolidaysNew.ics" and save it to the soplanning/holidays folder.
Then browse to "Import by country" and - voilà - you'll find it right on top of the list: