I deleted/disabled my calendar on Galaxy S3 I used the CleanMaster app on my Galaxy S3 to disable apps I do not use. 
I then accidently deleted my calendar. 
When I want to add an event, I get a noticification: 'You must at least have one calenar 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?
I had the same problem. It's easy to fix:


Go to settings/more/application manager
swipe left to get to "disabled"
Click on "calendar storage"
click "enable"


All your calendar entries should now re-appear.