How to use Event Calendar Extension with CiviEvents?
Follow the instructions here
https://github.com/osseed/com.osseed.eventcalendar/blob/master/README.txt

The latest release is here https://github.com/osseed/com.osseed.eventcalendar/releases/tag/2.0

It will not create the menu items automatically like it is supposed to, so you can create them yourself with the below instructions.  

Go to Administrator -> Customized Data and screen -> Navigation Menu -> Add New Item .
Set the title "Calendar Settings" and the URL civicrm/eventCalendarsettings Make the Parent Events.
Click save.

Go to Administrator -> Customized Data and screen -> Navigation Menu -> Add New Item . 
Set the title "Show Calendar" and the url civicrm/showevents.  Make the parent Events.
Click Save. 
Now the items are in the civi menu under events.