Heres a more practical approach to “US Holidays”. Have it contain just that - US HOLIDAYS and have any specidic religions add their own holidays to their IPhone. Rather than having various obscure religios holidays included AND NOT ABLE TO REMOVE.
iPhone SE, iOS 11
Disable the provided holiday calendar, and find and subscribe to a calendar with your preferred list of holidays.
Nobody will ever agree with which holidays are meaningless holidays, much less on which religions or religious holidays are or are not obscure, or otherwise.
Also consider an update to iOS 15 or iOS 18, depending on which iPhone SE in use here.
You are not talking to Apple here. If you want to send this to them, use the link below.
US HOLIDAYS include obscure observances
