For starters: See this knowledge base article: http://help.filemaker.com/app/answers/detail/a_id/3926/kw/weekend
To subtract out Holidays from such a total of week days, you can use a relationship such as:
YourTable::Date1 < Holidays::Date AND
YourTable::Date2 > Holidays::Date
To count the number of Holidays that fall on your date interval.
There are also ways to automatically update your Holidays table with Weekend Dates so that you just use the above relationship to adjust your elapsed time for days closed--whether holiday or weekend.