Calculating the Number of Work Days between two dates
For example, you may need to know how many business days fall in the month of November 2016. This calculation should exclude Saturdays, Sundays, and holidays.
The NETWORKDAYS function calculates the difference between two dates, excluding weekend days (Saturdays and Sundays).
As an option, you can specify a range of cells that contain the dates of holidays, which are also excluded. Excel has absolutely no way of determining which days are holidays, so you must provide this information in a range.
See also this tip in French: Calculer le nombre de jours ouvrés entre deux dates.