Comment # 4 on bug 1053947 from Wolfgang Bauer

Well then, what exact region have you configured? (might be a problem with this
particular one)

What do the following commands print (run as user):
grep Holidays ~/.config/korganizerrc
kreadconfig5 --file korganizerrc --group "Time & Date" --key Holidays

(In reply to Eli Wapniarski from comment #3)
> However, perhaps I have an upgrade hangover from kdepim 4 - 5?

I somehow doubt this, especially if you tried to change that setting afterwards
(it might be possible that the configured setting is not migrated correctly).

But you could try a fresh user account to rule that out.
(or delete/rename .config/korganizerrc)

> Perhaps I am missing some packages? or koganizer is looking for the holidays
> in the wrong location?

The holiday data files are part of the libKF5Holidays5 package which is
required.