After three years of avoiding it, I'm stuck upgrading to Plasma 5 again, and I've been tearing my hair out for the past three weeks trying to get things working like they used to. The #1 biggest dealbreaker is the calendar. If I can't get the calendar to work or somehow import one from another DE I shall have to switch to Trinity or maybe LXQt, and I really don't want to do that, as Plasma 5 has otherwise finally matured to be almost as usable as KDE4.

1) It is stuck on Gregorian when it should be Hebrew. I have Israel set as my region, that did nothing, I don't think any other country setting would do that and I don't see any explicit calendar system setting anywhere. Then I found a setting in a configuration file somewhere in ~/.kde/share/config/ that said "gregorian" and tried changing it to "hebrew," and apparently Plasma didn't like that because the line is gone now. In KDE4 I had two calendar widgets, one set to Hebrew and one to Gregorian, but there aren't any settings in the new widgets, only "Appearance" and "Keyboard Shortcuts."

2) Holidays are not marked. If I could at least get this working, it could be an acceptable workaround for #1, because at least I'd know when the month starts.

3) Weekends are not marked. Also in the Date and Time part of System Settings, the wrong dates are highlighted: the weekend should at least be Saturday not Sunday, and preferably also Friday. At least the week beginning on Sunday is correct.

4) Non-critical but annoying: The calendar seems to not respect my Oxygen theme selection, instead it's flat, extremely huge, and full of empty space like the new default themes are.

5) Very minor: The old fuzzy clock widget would display the time on a line above the date, neither the new one nor the new analogue clock can display the date at all.

6) Even less important, and was an issue in KDE4 as well: Why is the clock face tied to the theme? It seems silly to have to hunt down and install a whole theme just to change the clock face. As much as I love Oxygen I've always thought the decision to only have four dots around the edge of the clock face to be rather annoying. If I knew where to find the graphic I would like to just edit it.

I don't know what version exactly I have - some packages seem to be at 5.11 and others at 5.14 and still others at 5.54, and I'd be worried but I understand KDE is inconsistent about that. Devuan has available these packages at 5.15, 5.20, and 5.74; if upgrading can solve the issues I could pursue that, but I need to know before I do; data is very expensive where I live.

Alternatively, if there's some way to start a Trinity or LXQt panel on my KDE desktop I could just use their widgets. Trinity displays a Hebrew calendar just fine, and LXQt at least shows weekends.

Moved your post to the correct place.

Please provide some more information about your distribution and exact system version (the System Settings -> System Information, alternatively KInfoCenter has a copyable summary)

You moved it to kontact and PIM? But I do not use those? Kontact is not installed (I have no need for an address book) and I thought this was a system locale or UI issue, not a personal information issue?

I am running Devuan Beowulf, which I understand is mostly equivalent to Debian 10. I do not see a System Information under System Settings, but Info Centre has this:

Operating System: Devuan GNU/Linux 3
KDE Plasma Version: 5.14.5
Qt Version: 5.11.3
KDE Frameworks Version: 5.54.0
Kernel Version: 5.8.0-0.bpo.2-amd64
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7 CPU X 940 @ 2.13GHz
Memory: 23.4 GiB of RAM

I moved it because Holidays are not a System Setting, nor is the Hebrew calendar (or any other non-Gregorian calendar system), it totally depends on the user side calendar you use and whether the holidays are marked in it.
Basically, all computer systems use the ISO standard, you can read up about here: https://en.wikipedia.org/wiki/ISO_8601

What you remember as a setting was certainly not a system setting, but one in a user side calendar like Korganizer or whichever you preferred to use. (FWIW Korganizer was a standard installation in both KDE3 and KDE4).

I never used KOrganiser or any kind of full calendar application, either, I only ever interacted with calendars in KDE4 via the included Analog Clock and Fuzzy Clock widgets. Each of those widgets had a setting for calendar system, holiday selection, formats, etc., very similar to the Regional Settings section of the new System Settings.

If KOrganiser was not a hard dependency of anything I actually used, I may not have installed it before, either. I tend to look for "base" or "minimal" packages and add the bits I want rather than wasting disk space and network quotas with unnecessary software by grabbing a "standard" or "full" metapackage.

So, it sounds to me like where this information and capability was included by default in KDE4, you are saying that now I have to install and learn to use a full calendar application, manually add the information, and that will be reflected in the systems widgets?

What are your LOCALE settings? And do you have hdate or similar installed?

Omg I appreciate you and adore this!

Many have attempted, but all have fallen short. https://www.123calendars.com/june-calendar.html, which satisfies all of your requirements, was my favorite, and I owned it watch until it broke last year. I stopped using it ten years ago, but I kept it around since the concept was so excellent.