It is possible to have all non-working days highlighted in Leon duty sections and Planned Flights.
Go to Admin > Operator Settings > General and define it in Days Off.
Input a name (i.e. Bank Holiday) and a from-to period. By clicking an icon you can add more extra days off - they will all appear in green.
Crew Duties
Crew Activities
When you hover the mouse over the green field, the tooltip will open with the information what kind of day of it is.