There are two baked-in calendars that can't be deleted, Personal and Anniversaries... I can edit name/colour/default in both. Considering Anniversaries, it appears in the event edit pull down list only if toggled as default, but both states are unselectable. If I can't get rid of it, I'll use it, but it appears to be unavailable. Is there an explanation for this? Workaround is to ensure it's not default and ignore it in the "On this computer" calendars list.