Payroll

The cut-off was Thursday. The bank was shut.

Multi-country payroll fails on dates nobody in the room was responsible for knowing. Not the big holidays — the regional ones, the substitution days, and the half-day before the long weekend.

Where it actually goes wrong

The subdivision nobody checked. Asking a holiday library for GB returns the United Kingdom's national calendar. It does not return Scotland's. Run English and Scottish payroll off the same list and you will miss the 2nd of January — every year, silently, because nothing errors.
The weekend that is not the weekend. Friday is a rest day in Saudi Arabia and Sunday is a working day. A system that hardcodes Saturday–Sunday has the week exactly backwards for the office it is paying, and reports a clean run while doing it.
The holiday that moved. Some countries shift a holiday falling at a weekend to the following Monday; some do not. The substitution rule is the difference between a cut-off that clears and one that lands on a closed bank.
The half-day. Not a day off and not a working day. Treat it as either and the cut-off is wrong in one direction or the other.

What we hold that answers it

You needWe hold
Statutory closures per country 241 jurisdictions, as rules rather than a fixed list of dates — so next year is computed, not waiting to be typed in
Regional variation 510 subdivisions keep a date their parent does not — Bavaria, Catalunya, Scotland, and the several hundred others that diverge from the national list
The local working week Rest days where a jurisdiction evidences them, never assumed. Where we do not hold them, we decline to answer rather than defaulting to Monday–Friday
Substitution and in-lieu The rule that moves a holiday, recorded with the closure it applies to
Half-days Marked separately from full closures, at both jurisdiction and exchange level
Evidence you can show an auditor Every row carries the authority that set it, when we last checked, and how well it is evidenced — verified, corroborated or compiled

The field that matters to you specifically

Every date carries assurance. If a payment run cannot rest on an estimate, filter to what has been confirmed against the instrument:

GET /v1/calendar?jurisdictions=GB-SCT,DE-BY,SA
    &from=2027-01-01&to=2027-12-31&min_assurance=verified

The rest of the corpus does not disappear — it arrives labelled, so your system can treat a compiled date as a warning rather than a fact. That distinction is the entire product.

What we do not hold

Said here rather than discovered later:

Which product

If youTake
Put dates into a payroll system the Calendar API — scope is per territory, not per call, so cost tracks how many countries you run rather than how busy you are
Plan runs across offices by hand the Operating Calendar — free for two jurisdictions, no account
Need to hand something to audit or a client a Dossier — one country, sources and assurance on every row, edition on the cover

See a three-country run  Pricing