Deleting all iCal events and cleaning out iCal, so I can start new
Close Calendar/iCal. In Finder, press cmd + G. Type in "~/Library/Calendars". Press return. This folder contains all of the contents of your Calendar app on your Mac. Drag all of the contents to the Trash. Now launch Calendar/iCal again. It should be restored to its default settings.

Some caveats:


Ensure that you are not syncing calendars from iCloud, Gmail, Exchange, etc. (You can check in System Preferences or Calendar/iCal Preferences to be sure).
Do not empty the trash until you have ensured everything is working well. In a worst case scenario you can right click on items in the Trash and select "Put Back" to restore them if you want to undo all of this.