Open the Calendar app, then click File > Export > Calendar Archive… and save the file somewhere easy to find such as the Desktop or within Documents.
When you’ve completely closed the Calendar app, open a Finder window then in the menu bar click Go (then hold down the Option key) and click Library.
Sometimes when you attempt to empty the trash, you’ll receive an error message stating that the trash cannot be emptied because the file(s) are in use still. If you see this message then you’ll have to take an extra few steps to allow those files to be deleted.
First open up the Terminal application. You can find it in Applications > Utilities, or use spotlight to find it by pressing Command + Spacebar on the keyboard then typing in ‘Terminal‘ and pressing enter or clicking the app when it appears. Inside Terminal type in the following (or copy it from here) and press enter:
This will close any background processes still running for the Calendar application. After you enter the command above try emptying the trash again it should work now. If it doesn’t then reboot the computer and try again.
After the cache files have been successfully deleted, reboot your computer and relaunch the Calendar app.
At first it will probably load without any of your calendars visible - this is normal, just wait a few moments and it should update and the correct calendars should begin to show again. If your Calendar issue was related to a cache problem then it should now be resolved and working properly again.
That’s it, you should be all set now!
Note: If you need to restore your backed up calendar archive for any reason, you can do this from within the Calendar app by clicking in the menu bar File > Import… and selecting the backup file. Be careful with this though as it is easy to create many duplicate events this way if they already exist in the calendar. You should only need to do this if for some reason your calendar does not update properly after clearing the cache.