Keep iCal open without minimising it
You don't need iCal or Calendar to be open at all anymore. Open System Preferences and select Notifications. Then be sure to review Calendar settings and any Do Not Disturb settings.



Here is the Sierra article on how to use the notification center:


https://support.apple.com/kb/PH25142


You might put the Calendar at the top and remove any widgets you don't care for so they do not clutter up notifications.