From the Excel Help menu:
NETWORKDAYS
Returns the number of whole working days between start_date and end_date. Working days exclude weekends and any dates identified in holidays. Use NETWORKDAYS to calculate employee benefits that accrue based on the number of days worked during a specific term.
Syntax
NETWORKDAYS(start_date,end_date,holidays)
Important Dates should be entered by using the DATE function, or as results of other formulas or functions. For example, use DATE(2008,5,23) for the 23rd day of May, 2008. Problems can occur if dates are entered as text.
Start_date is a date that represents the start date.
End_date is a date that represents the end date.
Holidays is an optional range of one or more dates to exclude from the working calendar, such as state and federal holidays and floating holidays. The list can be either a range of cells that contains the dates or an array constant (array: Used to build single formulas that produce multiple results or that operate on a group of arguments that are arranged in rows and columns. An array range shares a common formula; an array constant is a group of constants used as an argument.) of the serial numbers that represent the dates.
If this function is not available, and returns the #NAME? error, install and load the Analysis ToolPak add-in.