It’s not always a computation problem. Unexpected bank holidays (happened to me in 2002 during a financial crisis), official days of mourning, etc. Nothing to

It’s not always a computation problem. Unexpected bank holidays (happened to me in 2002 during a financial crisis), official days of mourning, etc. Nothing to compute there: a government declares a holiday and you just enter it into the table. Sure, you can compute Easter (you can just look up Knuth’s explanation of the algorithm for this…), but you can’t compute government decrees.