How do I edit or delete the holidays that apple has decided to pre-load on my calendar?

Don't confuse the calendar application with the content, also called calendars. There are many of these available, for instance ones that will add all your favourite team's games to your diary. If you don't want this you don't subscribe to them. Similarly, there are some calendars subscribed by default. If that default one is not to your taste you can unsubscribe from it and then find another more to your liking and subscribe to it. Problem solved.

You can't edit the calendar but you can turn it off and then find one that better meets your needs. Go to the bottom of the calendar app and tap on "Calendars". Uncheck the U.S. Holidays app.

If you do a search for “remove Muslim holidays” you will find other threads that discuss the calendar issue. You either remove the Apple calendar and find something else or you ignore the parts you don’t like.

Elduker wrote:

I have not subscribed to any calendar via apple. However, I did purchase an item manufactured by apple that included a calendar. I find it too hard to believe that with all the technology and ability to customize and modify these items that the calendars could have been forgotten. I have faith in apple, it’s too much of a stretch to believe they’d force a calendar on customers littered with unneeded information while the rest of the calendar is so customizable.

Well i just looked at the calendar that my office purchased from Office Depot and it matches the calendar entries from my Apple iphone, so it's not only Apple. I don't celebrate the holiday, but I'm not going to cry about it, I just ignore it as another day.

The Apple calendar is a subscription service. You cannot edit it. If you want or need something different, you need to delete it and search in the App Store for a calendar that is better for you.