Do you mean you want to change the Holiday Subscription? None of the subscribed Calendars can be changed in the Calendar app. You can certainly subscribe to any Calendar you wish and that includes a Google Calendar subscription. Here is another subscription service for Calendars where you can subscribe to a predefines one or create your own Custom Calendar. The same rules apply though, you will not be able to edit the subscription in the Calendar app, any change in a Custom Calendar must happen from that source.
https://www.calendarlabs.com/ical-calendar/
As for the US Holiday Calendar that is on the iPhone by default, I agree with muguy that mine also lists Christian Holidays. This month is a perfect example where I see Palm Sunday, Good Friday, and Easter.
You can also skip the subscriptions altogether and create your very own Calendar with any event you want. These are editable and also can be set to be reoccurring so they will remain year after year.