When you add a public holiday to the calendar, you see the following error.
Public holiday is already happening for at least one holiday group on provided date
This error occurs because the public holiday group you've selected already has a holiday on the date you've selected.
For example, you’re adding a holiday on 26 December 2025, but you've already assigned Boxing Day to that group.
You can't have two different public holidays on the same date for a public holiday group. The solution is to unassign the interfering public holiday from that public holiday group.