Deleting Untitled Calendar from iOS Devices

Can‘t say specifically what to do but if I explain how I think the Calendar system works you may find a way yourself.

The Calendar App, both for macOS and iOS allows individual calendars to be created by the user which can be either in “On My Mac” or “iCloud”, the first is device specific and the latter will appear across all devices (ie sync). If you make changes to the first it will not effect other devices, but changes to the latter will filter through to all other devices connected to your Apple ID.

There is another type of calendar that can be created (or rather enabled) from within Preferences, namely Holidays, Birthdays and Siri Found in Apps, and these again are specific to that device.

Lastly, you can create a Subscribed calendar from web links that can either be in “On My Mac” or “iCloud”, again (and I think this is correct but haven't tried) the first is device specific but the latter will sync across all devices connect to your Apple ID.

The different types can be confusing but you may be able to follow through what you’ve done and correct the issue with the aid of the above.

That calendar used to be called US Holidays. Apparently Apple has renamed it to Untitled Calendar, because it has more than US Holidays. It also has Christian holidays, and non-holidays like April Fools Day, holidays that are celebrated but not "official", holidays celebrated by Americans who practice other religions, etc.