Not able to delete a birthday event from my calendar

Within Calendar, I see 4 calendars listed on the left side (1 - associated with my Gmail account, 2 - “Contacts”; 3 - “Family” and 4 - “Holidays in United States”).

I had a birth date listed for a person in my Contacts.  This birth date appears on my calendar.

I removed the birth date from the person’s record in “Contacts”.  However, the birth date still appears as an event in my calendar.

If I click on this birth date event in the calendar and then click “Delete”, the event disappears and within seconds re-appears.

When I go back to the person’s record in “Contacts”, there is no birth date listed!

What is going on?  How do I remove a person’s birth date so that it no longer appears on the calendar?