First of all, iPhone seems to have an issue on showing all-day events on the Calendar widget (haven't found a workaround that doesn't need to install other app). So I thought, well, I'll just add a reminder to all events and it's done.
Of course, I went to Calendar setting and for Default Alert Times I deactivated them: None.
When I go to an event and add a reminder, it won't show in the widget. Then, it will automatically change the reminder to none by itself. So, it seems it doesn't work when I set up a reminder (like 2 days before) and then it also changes the reminder to none.
You can see a video of this here: https://www.dropbox.com/s/o1xafro8agmbg3q/video-1547901236.mp4?dl=0
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