Samsung
Show or Hide Public Holidays in Calendar auf Samsung
Public holidays in the Samsung Calendar are not added manually. They come from a synced Google Account, and the country whose holidays get shown depends on the region or language selected in your system preferences.
This is why switching your language or region can suddenly change which holidays appear, even if you never touched the calendar settings. The holiday list itself is not stored locally, it is a subscription tied to that account.
To hide them, you need to disable the holiday calendar for that specific account rather than deleting single entries, since new ones would just sync back in.
Menu path
Tap on Calendar ➔ Open the Menu ➔ Turn On/Off Holidays of the respective account
If you use more than one Google Account, each one has its own holiday toggle. Turning off holidays for one account does not affect the others.
If you actually want holidays from a different country, changing the region in system settings is usually a better fix than trying to add a second calendar manually.