Hello tjeerd181,
We see you have a question about calendar events. The steps below cover how you'd add holiday events: Use the Holidays calendar on iPhone - Apple Support
"Change your primary holiday calendar to a different region
- Go to Settings
> General > Language & Region, then tap Region. - Tap a region, then tap Change to [region].
- In Calendar, tap Calendars at the bottom of the screen.
- Deselect the Holidays calendar, then select it again.
Note: To subscribe to a holiday calendar from a different region, see Set up multiple calendars on iPhone."
It sounds like you might've already tried this. If you have, and you've also covered the steps in the secondary link above, we recommend contacting us directly: Apple Support
Kind regards.