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From Apple web support​

Use the Holidays calendar on iPhone​

In the Calendar app
37f172f1c510ca995502e1b99d4b0862.png
, the Holidays calendar displays holidays based on the region set on your iPhone.

Note: The Holidays calendar is a subscription calendar. You can’t add or delete holidays, because subscription calendars can be modified only by the calendar provider.

Show or hide holidays​

  1. Tap Calendars at the bottom of the screen.
  2. Select or deselect US Holidays (or the holiday calendar for your country or region).

Add a holiday calendar from a different region​

  1. Tap Calendars at the bottom of the screen.
  2. Tap Add Calendar, then tap Add Holiday Calendar.
  3. Tap the holiday calendar you want to subscribe to, then tap Add.
To change settings, like notification alerts or the name of a holiday calendar you’ve subscribed to, see Set up multiple calendars on iPhone.

Apple support article holidays
 
I guess it is called important dates calendar (

2024 non-federal holidays​

) and US holidays just include major holidays
 
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