how do I delete a read only calendar entry

It's not a secret at all. You can disable the Birthdays calendar in the Calendar preferences window, or you can delete the Birthday property from contacts whose birthdays you don't want to be reminded of.

So, the solution is to delete a contact just because you don't want to be notified (via Apple's connected calendar, that doesn't let you delete "Birthdays") of the contact's birthday every year?

And for those of us who have thousands of contacts whose birthdays we don't care to be notified about? Should we manually delete or is there a button preset for deleting most of our contacts?

Found some information for google calendars .

If you would tell us where this calendar is housed then it may be possible to frame an answer

The Birthdays calendar is generated automatically from the Contacts database. You can't modify it directly.

If it is the Birthdays calendar then you control entries on it from the Contacts app.

If you enter a birthday on a contact card it will show a reminder in calendar.