Namedays Calendar
Provides namedays for multiple countries

Database of ICS files for non working days.
Non-Working Days API is a free Calendar API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Open repository of non-working days ics files.
Details from github.com.

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Provides namedays for multiple countries
Simple REST API for checking working, non-working or short days for Russia, CIS, USA and other
The Non-Working Days API is provided by gadael (github.com). Open repository of non-working days ics files.
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.
CORS support for the Non-Working Days API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Non-Working Days API is served over HTTPS, so it's safe to use in production.