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 set up the days you want under Holidays / Days off.
You can add more days by clicking on the green + icon, or remove a day by selecting the line and clicking on the red X icon to delete.
The holiday will then be displayed as shaded in resource planner.
You must also enable "Show holidays in the calendar" if it is not already enabled in the settings
You will find the settings for resource planner by selecting the 'Settings' icon in the top toolbar