That's a bold move prc62; and one that runs totally contrary to Apple forcing us to move all of our data into iCloud.
I was guessing that was the source of the problem; but with no other solution, I may just follow your example.
When I upgraded to Mountain Lion, it appears that a whole lot of my calendars were sucked into iCloud whether I wanted that or not. I've always preferred to have my Calendars On My Mac; because there have been numerous other problems over the years when that was not the case.
BTW, for others with this problem.. with 10.8 (Mountain Lion) Calendar issues this message whenever I add an .ics-based event (more likely for me from a web page such as a webinar reminder, than an emailed event)...
"Calendar can't read this Calendar file. No events have been added to your calendar." If I open the .ics files in a text editor, they look perfectly normal.
I've been soooo disappointed in what's happened to Apple's core apps with each "improvement". Functionality disappears (iCal/Address Book), the app gets uglier and harder to use (again Calendar/Contacts), huge amounts of data couldn't be imported to the "upgraded" app from the previous version (Mail), and so on.
If Apple can't genuinely improve these apps, I wish they'd leave them alone instead of crippling them.
Obviously this rant isn't going just to you prc62. It's a general problem.