Set background color for office/working hours
To have the ability to set a different background color for office/working hours.
Easier to know if the appointment or event falls within office/working hours at a glance.
Easier to see the available timeslot within or outside of office/working hours when setting a appointment or event.
Stefan Lohwieser commented
And not only for working hours, also for special occasions like holidays, vacation and so on.
Easiest way in my opinion could be the choice, if a calendar entry is displayed as standard entry or as backgroud entry (like the automated color for Sundays)