Holidays calendar not showing in macOS Calendar app
https://support.apple.com/en-bh/HT208541 suggests: The Calendar app on your iOS device and Mac features a built-in holiday calendar that’s customized based on the holidays that are most recognized in your country or region...
Anybody knows why the Holidays Calendar is not showing in macOS (and other Apple devices) anymore? - although checked in Calendar->General->Show Holidays Calendar
BTW: Unchecking and checking the Show Holidays Calendar results an automatic unchecking of Show Birthday Calendar...
Updates (15.4.) did not solve the issue. Ohhh - Catalina