Thousands of people post every day in this user-to-user forum without any problem, and get the answers they need. I’m sorry you found it so much more onerous than those thousands of other users.
The answer is that the holiday calendar is a subscribed calendar. That means it is “owned” by the entity that created it; it doesn’t exist on your phone, it exists on its creator's server and is displayed on your phone, and can only be edited by its creator.
There is no such thing as a “countrywide legal holiday”. In the US there are NO legal holidays; there are holidays that are specific to certain states, and holidays where US Government employees get the day off, but those only apply to government employees, and holidays that certain religions consider to be holidays. Apple’s holiday calendar includes the Federal employee days off, a few state holidays, a few holidays that are important to certain ethnic groups (Cinco de Mayo, for example, which celebrates Mexico’s defeat of France in 1862 and is informally Mexico’s independence day), and the major holidays of the 5 most widely practiced religions in the US.
If the Apple holiday calendar doesn’t suit you just uncheck it. You can then create your own holiday entries, or install one of the several thousand other published holiday calendars available on the Internet.
Here are a few of sources for holiday calendars: