bizday_of_period() to compute the business day of a date within a given period (e.g. date xxx is the 3rd business day of the month according to the UnitedStates QuantLib calendar)
is_bizday(), bizdays_between() which wrap similar qlcal functions, but allow evaluation in a specified QuantLib calendar without making persistent changes to the globally configured calendar.
with_calendar() and local_calendar() which facilitate temporary changes to the configured qlcal QuantLib calendar a’la with_*() and local_*() functions from the withr package.
set_calendar() which changes the globally configured qlcal QuantLib calendar, but only if the specified calendar is valid and different from the currently configured calendar.