Non-Working Days vs UK Bank Holidays

isdayoff.ru answers one narrow question extremely cheaply: for a date or date range, is it a working day, a weekend, or an official non-working holiday. Responses are bare digit strings (0/1 per day, or 2 for a shortened day) rather than JSON, which keeps payloads tiny for the batch/range endpoint. It documents Russia, Belarus, Ukraine, Kazakhstan, and the US as supported country codes, but live probes this run found only Russia (the default), Belarus, and Kazakhstan return genuinely varying data — the Ukraine and US codes are accepted (HTTP 200) but answered every single day of several different months with a flat, undifferentiated "0". It's free, keyless, and has no published rate limit, built for production-calendar bookkeeping rather than general holiday metadata.

The UK government publishes England & Wales, Scotland, and Northern Ireland bank holidays as a single static JSON file at gov.uk, listing each holiday's date, title, and whether it's a “substitute day” moved from a weekend. It's official, authoritative, and requires nothing beyond an HTTP GET — no key, no rate limit, and it's licensed under the Open Government Licence for reuse including commercially.