iPhone holiday calendar off by 1 day The built-in holiday calendar on my iPhone 8 running iOS 12.4.1 is off by one day. For example, it shows New Year's Eve on December 30 and not on December 31.
So far I tried to :

Enable automatic date &amp; time in Settings &gt; General &gt; Date &amp; Time &gt; Set Automatically = &quot;On&quot;
Desable the time zone override in Settings &gt; Calendar&gt; Time Zone Override = &quot;Off&quot;
Reset the iPhone
Switching off automatic date &amp; time and entering my time zone then switching back to automatic
Switching off Location Services and GPS in Settings &gt; Privacy &gt; Location Services. then switching back on

What could I do next ?
Note: My personal calendar doesn't have the issue nor my MBP's calendar ; but  I added another location holiday and it have the same issue...
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.