Quit Calendar. Triple-click anywhere in the line below on this page to select it:
~/Library/Calendars/Calendar Cache
Right-click or
control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A Finder window should open with a file named "Calendar Cache" selected.
Move the selected file to the Trash. There may be one or two other files in the same folder with names that begin in "Calendar Cache". If so, delete those files too.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination
command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.