Business days here are Monday to Friday. The calculator walks through every day from the start date up to (but not including) the end date, counts weekdays, and subtracts any holidays you list that fall on a weekday in that range. Tick the box to count the end date as well.
A normal year has 260 or 261 weekdays. Most countries have several public holidays on top of that, so 250 or fewer working days is typical.
Thirty calendar days contain about 21 or 22 business days, depending on which weekday you start.
Only the ones you paste in. Holidays differ by country and region, so add your own dates, one per line.
See also: Days Between Dates · Add Days to a Date · Hours Calculator