All-day events and events ending at midnight create duplicates of days in the sidebar
Basically what you can see with this screenshot of GNOME Calendar 43 on Fedora 37: today, "samedi octobre 22" is shown twice instead of only once.
I can only guess that there's a check going on for event boundaries at midnight, but when you have an event that ends exactly at midnight (like my "prep une autre DOUBLE batch de pâte à pain" event, from 23h00 to 00h00) or an all-day event (which, I will guess, is considered to be an event that goes from midnight to midnight?), then GNOME Calendar thinks "Oh it's spanning two days, from the 22nd to the 22nd! Clearly not the same as those hourly events that occur during the day of the 22nd! Yup, makes sense!"
This can lead to some funky situations such as this (screenshot taken on a 31st of August):
See also issue #968 ...