iPhone holiday calendar off by 1 day
What if you unsubscribe from the holiday calendar and then set the time zone on your phone to be exactly what you want.

Then power cycle it (to be sure all the apps know and can read the correct zone).

When you subscribe back to the calendar - pay attention if there are advanced options on time zone.

We might need to know the exact subscription URL you're using and which account backs this calendar. iOS calendars can be stores in Exchange, O365, google, iCloud or other third and first party solutions or not sync at all. Depending on your specific set up, the time zone settings might not even be on your phone if you're syncing down from a cloud service.