Calendar

You are subscribed to the Apple Holidays calendar for the region you have registered in; it’s a built-in feature. You can uncheck it in the Calendar app by tapping on Calendars, then you won’t see it.

Uncheck the holidays calendar you are currently using and use your favorite search engine to find a holidays subscription calendar that meets your needs. Or turn off the calendar subscription and manually enter only the holidays you want.

You cannot edit a calendar you subscribe to.

Yes, there is no native holidays within the Calendar app but the phone does come with the US Holidays subscription calendar checked. Open the Calendars app, at the bottom in the middle tap on Calendars, and in the next page uncheck US Holidays or whatever one you have.