iWork Numbers: Can i count the number of Mondays in a specific month with a formula?

Hi

I am looking for a streamlined way of calculating an amount to invoice for a job that I am paid for weekly, but for which I invoice monthly, meaning that I am invoicing different amounts each month, depending on how many Mondays fall in a given month (i.e. 4 or 5).

So I am wondering if there is a function I can use to calculate the number of Mondays in a given month automatically and then I can use this value to multiply by my weekly charge to calculate the amount to invoice each month.

Thanks

Nick