Holidays are displayed on the Samsung Calendar by a signed in Google Account, and the country of the displayed holidays depends on the region or language selected in System Preferences.

If you want to hide the holidays, you'll need to do so in the display options for that account. This guide shows how to hide/show the holidays from your Google Account. Other accounts should be handled in the same way.

Android 10 (One UI 2.0)
Step 1:  Open the  Calendar -App
Step 2:  Open the  menu
Step 3:  Tap on desired  Account
Step 4:  Enable or disable  Holidays in xxx
  1. Open the Calendar-App
  2. Open the menu
  3. Tap on desired Account
  4. Enable or disable Holidays in xxx

Samsung Instructions