The download of the app doesn't contain viruses or any kind of malware. Phillip Steffensen is the official developer of Deutsche Feiertage & Ferien that can be found in the Productivity category. The tool was downloaded and installed by over 0,5M users and the latest stable release was on Nov 2, 2019.
On Google Play, this application has been rated 4.4 by its users. Pay attention to its terms of use because there are in-app purchases and ads. Samsung and HTC users install the app quite often.
It's free of charge to get the installation package of Deutsche Feiertage & Ferien, also, you can have a look at users' reviews on Droid Informer.