I presume you are talking about discounting weekends and public holidays and not a Soviet calendar.
I don't know enough about Boost's calendar and date_time facilities to give a very good answer, but it seems to me that it would be a fairly simple calculation to make on your own.
Given a begin date and and end date, find the total number of weeks that elapse. Subtract twice that from your days. Subtract an extra day or two when the start and end dates are weekends.
Then you will have to do some special counting for non-weekend holidays. You'll have to generate a list of holiday dates in the given range for that. (The vast majority of holiday dates are easy to calculate -- even Easter. For the really obnoxious ones you'll need a historical archive.)