bug#43677: 27.1; Calc holidays off-by-one error, incorrect business day calculation

[Mattias Engdegård]

> Indeed -- applying the following patch (which numbers Monday as 0 and Sunday 
> as 6) makes the test case work correctly for me. But trying to follow the 
> code makes it unclear... er... what's going on, and there aren't many 
> comments in the Calc code to explain what they were thinking.

Welcome to Calc!

> I wondered whether this might be a US/non-US thing (with a difference in what 
> day starts the week), but I couldn't really find anything in calc talking 
> about that... 

The weekday numbering can be arbitrary as long as it is only used internally. 
Calc's date representation (from Rata Die) sets Dec 31, 1 B.C. as day 0, and 
this happens to be a Sunday. That means that (mod DAY 7) is an easy way to get 
the week-starts-on-Sunday numbering used by some foreigners.

Since the bug is a regression -- it is present in Emacs 26.1 but not in 22.1.1 
-- it may be a good idea to find out exactly what change introduced it, and 
work from there. That might reduce the risk of causing secondary bugs.