Comment # 3 on bug 1159967 from Wolfgang Bauer

(In reply to Eli Wapniarski from comment #2)
> Be that as it may, I also mentioned that Israel Holidays do not appear in
> the calendar.
> 
> I should have been more clear about that, I guess. I am able to select
> Israel Holidays
> 
> Settings -> Configure Korganizer -> Time and Date -> Holidays
> 
> I quit Kontact and restart it, but the Holidays do not show up

And that's exactly what was originally reported in
https://bugs.kde.org/show_bug.cgi?id=383896 (which I only filed because an
earlier openSUSE bugreport by you in case you don't remember).

AIUI, the underlying reason is the same though: there's no support for the
Hebrew calendar (or actually any other than Gregorian, it seems) in Qt5/KF5.

The latest Qt 5.14 has a change that sounds promising, so it may be possible to
fix that in the future:
QCalendar adds support for Calendars other than Gregorian, by implementation of
suitable back-ends.
Back-ends to implement the Gregorian, Jalali (Persian), Islamic Civil,
Milankovic, and Julian calendars. We look forward to other contributions.

That's out of scope for Leap 15.1 unfortunately though, and will take a while
to  land in Leap anyway (Leap 16 maybe?).
And of course, korganizer (or whatever component is responsible here) would
need to adjusted for that, and this is something that needs to be done
upstream.