It is now possible to set up holidays and days off in the Resource planner to mark that this is not a workday.
Set up for this is done in Configuration - Company - Time zones
You select the day under Holidays / Days off
You can add more days by clicking the green plus icon, or remove a day by selecting the line and clicking the red x-icon for deleting.
The holiday will then be displayed as shaded in resource planner.
You also need to enable to "Show holidays in the calendar" if that is not already enabled in your settings
You will find the settings for resource planner by selecting the settings icon in the top toolbar