That sounds super frustrating. It may be an issue with a corrupt preference or cache file. Try this:
1. Quit iCal
2. Open Finder
3. Access your Home Library
- If you have 10.7 or later, the Home Library is hidden. To access it, click the Go menu at the top of the screen and then hold down the Option key. Library should appear. Click on it while holding Option. Once it is open you can let go of Option.
- If you have 10.6 or earlier, click on the Go menu at the top and click on Home. Then in the Finder window, you should see the Library folder. Click on that.
4. In the Library folder, look for Caches and open that folder. In here, we want to delete the com.apple.iCal folder. (You can temporarily move it to the Desktop till you resolve the issue before you move it to the Trash).
5. Go back one folder to the Library folder, look for Preferences and open that folder. In here, we want to delete the files that start with com.apple.iCal. (Again, you can temporarily move it to the Desktop till you resolve the issue before you move it to the Trash).
6. If you are running 10.7 or later, go back one folder to the Library folder, look for Saved Application State and open that folder. In here, we want to delete the folders that start with com.apple.iCal.
7. Restart your computer.
8. Try iCal again
If everything is working again, you can move the files that you moved to the Desktop to the Trash.
I hope this helps!