This doesn’t actually solve the problem because the problem is that the list of holidays depends not only on the country but also on the particular company you’re interacting with. So, is_holiday is something like a function of the parties involved in a particular contract.
There is no general problem. There is the act of doing business between parties. You do this across N relationships. You have N vendor contacts. You document them. You deal with them. This is what working with someone is like! Of course after doing the same thing 3 or 4 times you might understand some patterns, but even if you don't totally factor it out you'll at least have a general pattern.
The point is you don't write the is_holiday function, you might write "is_jp_govt_holiday" or "is_mufj_bank_holiday" (though really it might be more "is_mufj_bank_transfer_csv_acceptance_day" or whatever), that might look at "is_jp_govt_holiday". You write functions to their contexts, in a specific way. This lets you write "hacks" that aren't hacks when you get the specific email. A bunch of tiny, specific functions calling other tiny, specific functions.
5-word functions might look gnarly but they will be specific, and you got the magic of autocomplete to save you some typing.
