Non-Working Days vs The Calendar

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 Calendar publishes pre-generated, static JSON (and downloadable .ics) files for US federal and all-50-state holidays (2025–2028), 30 countries' public holidays, plus FOMC meeting dates, US tax deadlines, market holidays, and key-date calendars for a dozen sports leagues. Every one of its 99 documented endpoints is a flat file behind Cloudflare, so there's no authentication, no rate limit, and CORS is wide open — licensed CC BY 4.0 as long as you link back.