These are the days in the year that are non-school days (besides school holidays). E.g. Public holidays outside of school holidays or specific pupil free days.
To add a Non-School Day
Click the Non-school Days tab.
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Click Add New Non-School Day.
Select the date, add a description and select which year groups it applies to.
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Once completed click the tick icon.
You can now add classes to your timetable. See Adding Classes to a Period.