If I open the calendar from the panel (clock), then the dates display a triangle in the corner indicating that there is an event on that day (a public holiday or an event from the personal calendar), but writes from the side that "there are no events on this day." In order to see the event on the side, you need to choose another day, and then again the desired one. After this event is displayed correctly.
I recorded a video for you: https://www.youtube.com/watch?v=Bl6wvhSXnfI
KDE Frameworks 5.57
STEPS TO REPRODUCE
1. Click on clock after login on OS
move to product plasmashell::clock
Same problem here.
Operating System: Fedora 30
KDE Plasma Version: 5.15.5
KDE Frameworks Version: 5.59.0
Qt Version: 5.12.4
Kernel Version: 5.1.16-300.fc30.x86_64
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-6700HQ CPU @ 2.60GHz
Memory: 15,4 ГиБ
Can confirm on Manjaro, plasma 5.16.4.
*** This bug has been marked as a duplicate of bug 395956 ***