I work on up-to-date system.
I checked Holidays in “Available calendar plugins” and in “Holidays” option I selected my country/area as “pl_pl”. I pressed Apply and OK button. Unfortunately checking every month in calendar I didn’t notice marked any holiday. To be sure I selected also UnitedStates country (us_us), later Apply and OK. Also after that nothing changed. OK. I restarted system and I got the same result :(.
I have installed kholidays in version 16.04.2-2. I have also installed: perl-datetime-event-ical

I wonder why the holidays function doesn’t work, whereas were mentioned with release of Plasma 5.7. Maybe I don’t have installed some library or some other package. Could someone knows where could be the problem?

I also tested it in KaOS with the same result :(.