The Analysis Toolpack provides the function NETWORKDAYS to find the number of working days between two dates. From Help:
To find the number of working days in a month, given any day in that month, you can use this formula
To get the start date, the DATE function is used with the month and year of the given date and 1 for the day argument. For the end date, the DATE function is again used. The 0th (zeroth) day of the next month is the last day of the current month, so 1 is added to the month argument and zero is used for the day. The Holiday range is reference and, being in the US, I have only one holiday this month.