Clear the macOS Calendar Cache

If your Calendar application is not synchronizing properly, clearing the Calendar cache will force the Calendar app to clear all your calendar entries. This does not delete them from the cloud services (iCloud, Office365, Gmail). It merely forces your Calendar app to clear out all the entries the application has stored in its memory and start fresh. 

•First, Quit the Calendar application.

•Go to the Finder menu...and select Go whilst pressing down the Option key this will give you an additional option on the submenu called Library.

•Select Library which will open another folder

•Within the Library folder, there should be a Calendar folder, double click on it to enter the folder.

•Within Calendars, typically you'll find 3 files made today titled:

Calendar Cache

Calendar Cache-shm

Calendar Cache-wal

•Select and drag these 3 files to the Trash.

•Start the Calendar application

Slowly, your calendar and calendar entries should begin resynchronizing. You may have to restart the computer if the calendar accounts do not come back in a minute or so.