How to Add Other Holiday Calendars to iCloud Calendar
Instructions
Calendars are synchronized across devices bound to same account. From any of the devices you can add the new holiday calendar. Just be mindful that it will not show up on the web version of iCloud.
- On macOS Calendar app, under File > New Holiday Calendar, search for the calendar.
- On iOS Calendar app, from Calendars > Add Calendar > Add Holiday Calendar, search for the calendar.
As far as I am aware, iCloud web doesn’t support adding a holiday calendar or displays the subscribed holiday calendars so it has to be done on the device apps.