Working days: weekdays excluding holidays.

Calendar days: actual days.

as per my almost-half-century on the planet (at least in English).

Cannot see why anyone would use calendar days. Unless they were trying to distinguish from working days.

So yes that would mean weekend and bank holidays.

They're definitely thinking of working days, calendar days are days on a calendar.

Isn't every day is a calendar day?

Calendar days to me would be actual days, I.e. including weekends and bank holidays. There is no way it can be the same as working days!