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. Warning: if you have an On My Mac calendar, you may not want to do this, since as of this writing we are unclear as to how this will affect those entries.
•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