Why do some Events require "Bind Event" and others not?

I am not clear on the rules on which events can be “implemented” directly in blueprints and which events need to be bind to a custom event first and then can only be implemented from that custom event:

If you look at this: We have 2 events. Calendar.NewDayBegan and Button_0.OnClicked

Both are declared the same way ( DECLARE_DYNAMIC_MULTICAST_DELEGATE + BlueprintAssignable) and broadcasted from c++. Both are implemented the same way: Click on the Calendar or Button in the variable view, click on the event “bind”. But the Calendar does not work, the Button does work.

The only way to get the Calendar event to work is to bind it like displayed.

But why does it work for the one type and not for the other? And is there a way to make the Calendar.NewDayBegan work like the Button_0.OnClicked ?