An exception overrides the weekly pattern for one specific date, in either direction.
A closure
Mark the date as non-working — public holidays, a shutdown period, an office move. The date then contributes no working time and calculations skip over it.
A special working day
Mark the date as working and give it its own hours — a Saturday opening, a shortened day before a holiday, cover during an unusual period.
Each exception can carry a note. Use it: "August bank holiday" is far more helpful to whoever maintains this next year than a bare date.
This is the part that gets forgotten
A calendar with last year's holidays quietly produces wrong due dates every time one of those dates comes round. Nobody notices until a customer points out that a target was missed over Christmas — by which time you have a month of incorrect measurements behind you.
- Load next year's public holidays as an annual task, with a reminder in somebody's calendar.
- Load them into every calendar, not just the main one.
- Remember that different countries have different holidays if you operate in more than one.
- Do it in advance. A holiday added after the event does not retrospectively correct due dates already calculated.