'You must have at least one calendar activated under My calendars. Do you want to open the calendar list?' after disabling/deleting calendar app

I used the CleanMaster app on my Samsung Galaxy S3 to disable apps I do not use. I then accidentally deleted my calendar. When I want to add an event, I get a notification: > You must have at least one calendar activated under My calendars. Do you want to open the calendar list? When I hit OK, I just get the blank calendar list. I have now also downloaded the 'Business Calendar' app - but this also notifies me that I have no calendars selected. How do I reinstall or reactive my calendar then?