Very nice article Alex, thank you.
Indeed IsBusinessDate 'Y' or 'N' is great for building an understanding of the concepts, I remember when I implemented this functionality using something similar; I used the name of the holiday in the HolidayName field ('Sunday', 'Saturday', 'Easter', 'Independence day', etc. ) and considering as business day the records where HolidayName is null or empty.
This helped me when I was asked "why there are less working days this month?" :-), and I had the answer at hand. Also it was easy for the HR to define company's holidays as they might change during the year, by just typing the name of the holiday in the interface.
I am curious in approaching multiple calendars, i.e. for different departments or for different geographies.
For example: National days are different for most of the countries but also some religious holidays, Italian branch of one company has the Easter holiday in another period of time than Russian branch.
Also for production department some Saturdays might be business/working days since for sales dept. Saturdays might be weekends/non business days.