Try removing the com.apple.iCal.plist file from your Macintosh HD/Users/yourusername/Library/Preferences Folder. To find that Library folder, use Finder>Go Menu>Depress the "Option" key>Library. Drag the .plist file to your desktop, and log out/in or restart.
Next, make sure that you are not connected to the internet. Then go to your Macintosh HD/Users/yourusername/Library/Calendars Folder and remove any files with "Cache" as a part of the file name. Use the same method as listed above to get to the correct Library Folder. Log out/in, or restart and check iCal for functionality.
I've been having the same problem. I tried Ferd II's suggestions and they didn't work. I found a link that suggested I delete all the calendars and start over which I did. Still no good. Then I decided (for other reasons) to do a completely fresh install of the OS and all my apps. Still no good.
Calendar is usable but only about a minute or two after I have opened it. It always begins to update my calendars and then the beach ball spins for 60 seconds or so.
This is extremely frustrating.
So, after doing some experimenting I think I've narrowed this down to a network problem for me. It seems Calendar always spins for half a second when it opens - not sure what it's doing - but on a stable, speedy network it only spins for a moment. On the slow, unstable 3G network I've been using most of the time lately it spins for much much longer - this happens even when I disable all calendars.
So, perhaps this is the problem. I don't really understand it but it's the best I can come up with right now.
Been having the same problem since I upgraded to Mountian Lion and just called Apple about it. The advisor recommended resetting my pRAM by holding down command+option+p+r while restarting and waiting for the chime twice. This helped speed it up a little but still have to wait for the spinning rainbow for about 5-10 seconds after opening iCAL. The apple guy said that this is because in Mountain Lion, iCAL access more info in iCloud than it did with previous versions of OSX.
Hope this helps,
Just thought I'd throw in my personal experience here.
I was having this exact same issue. Upon launch, iCal would freeze for a solid 60 seconds before letting me do anything. After reading this thread, I decided to simply delete my Microsoft Exchange account from iCal. All that was left was iCloud. The problem disappeared completely. No freezing at all. I never messed with any .plists either.
I'll echo the Exchange account soulution. Problems began when I coonected my Exchange account and received an invitation via Exchange. Attempting accept the invitation created a situation where windows in iCal would not close and the entire system slowed. The activity monitor showed 98% of the cpu time being used by the calendar agent with nothing apparently going on.
Removing the Exchange account caused the iCal External Sync agent to capitalize the CPU in activity monitor for a few minutes, then everything in the monitor was back to normal. I quit iCal and restarted the machine. All is good again.
Now, if I could just get the hpdot4d problem fixed...