SUMMARY

Any selected holidays are _missing_ in the month view for all those times which are defined as "Working Period" in the "Time and Date" settings.

In other words: The month view only shows holidays if they occur on days which are _not_ a "Working Period" per "Time and Date" settings.

STEPS TO REPRODUCE
1. In Settings -> Time and Date -> Holidays, choose some Holidays (tested for de_de).
2. In the month view, holidays will appear as expected.
3. In Settings -> Time and Date -> Working Period, disable "Exclude holidays".
4. In the month view, all holidays on weekdays will be missing.
5. In Settings -> Time and Date -> Working Period, additionally include Saturdays and Sundays as Working Period.
6. In the month view, all holidays will be missing.

OBSERVED RESULT
The month view shows holidays if (and only if) they fall outside of the defined "Working Period", which can be the case for two reasons:
a) either "Exclude holidays" is activated, or
b) the corresponding weekday is disabled in the "Working Period" definition.

EXPECTED RESULT
The month view should always show all selected holidays, regardless of the "Working Period" definition.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 21.10
KDE Plasma Version: 5.22.5
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2 (built against 5.15.2)