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.
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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.