Use "Find and resolve duplicate fonts" in Mac 101: Font Book.
Also, quit iCal, and try removing the com.apple.iCal.plist file from your Macintosh HD/Users/yourusername/Library/Preferences Folder. Use the Finder>Go Menu>Depress the "Option" key>Library. Drag the .plist file to your desktop, and log out/in or restart and check iCal for functionality.
This helped put me on the correct track. I use font explorer to enable fonts and after disabling all but the system fonts the issue went away.
Now I will just have to find the conflict.
Thanks for the tip to use font book and although I couldn't find "Find and resolve duplicate fonts" I was able to use "Look for enabled duplicates".