Hullo Steven,

I did this (in DOS days) by having the user input holiday dates into a .dbf file, then automatically adding Saturdays and Sundays (or others) to the file up to the last holiday date entered. The file must then be sorted into date order and indexed on the date.
To find the working days between two dates, subtract earlier from later and then further subtract the difference in the record numbers found by seeking each date (with near on) to eliminate the non working days, adding the odd one somewhere for inclusive/exclusive periods.

Good luck.