Simple REST API for checking working, non-working or short days for Russia, CIS, USA and other.
Non-Working Days API is a free Calendar API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
API производственного календаря
Details from isdayoff.ru.

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Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Provides namedays for multiple countries
Public holidays for US states and 30 countries plus sports and finance calendars as static JSON
The Non-Working Days API is provided by isDayOff() (isdayoff.ru). API производственного календаря
Yes. The Non-Working Days API is listed in the Calendar category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Non-Working Days API requires no authentication, so you can start making requests without signing up.
Yes. The Non-Working Days API supports CORS, so front-end JavaScript can call it directly.
Yes. The Non-Working Days API is served over HTTPS, so it's safe to use in production.