What is the difference between calendar days, working days and business days?
Calendar days include every day on the calendar, from Monday to Sunday. Working days usually refer to the days actually worked, often Monday to Friday. Business or open days are days when an activity can usually take place, often excluding weekly rest days and some public holidays.
Explanation
These three notions look similar, but they are not used in the same way.
Calendar days are the simplest: every passing day is counted, including weekends. A period from the 1st to the 10th therefore represents 9 raw days of difference, or 10 days if the end date is included.
Working days usually refer to the days actually worked in a standard organisation. In many cases, they are Monday to Friday. This is useful for estimating processing time, delivery time or actual work duration.
Business or open days are broader: they refer to days when an activity can usually take place, often Monday to Saturday, excluding public holidays or specific rest days. Depending on the context, a Saturday may be a business day without being a working day.
The key point is to check the wording used in the document, contract or rule you are applying. Confusing “working day” and “business day” can shift a deadline by several days.
Concrete example
For a period from Monday to Sunday:
- calendar days: 7 days;
- standard working days: 5 days, Monday to Friday;
- standard business/open days: often 6 days, Monday to Saturday, excluding public holidays.
This is not just administrative wording. Depending on the term used, the result can change.
Common mistake
A common mistake is assuming that working days and business days always mean the same thing. They do not. For an important calculation, always check the definition that applies to the context.
Sources & methodology
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