Public holidays for US states and 30 countries plus sports and finance calendars as static JSON.
The Calendar 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.
Free static JSON API: US/world public holidays, FOMC dates, sports and finance calendars. CORS-open, no auth, no rate limit.
Details from the-calendar.net.

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Simple REST API for checking working, non-working or short days for Russia, CIS, USA and other
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The The Calendar API is provided by the-calendar.net (the-calendar.net). Free static JSON API: US/world public holidays, FOMC dates, sports and finance calendars. CORS-open, no auth, no rate limit.
Yes. The The Calendar 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 The Calendar API requires no authentication, so you can start making requests without signing up.
Yes. The The Calendar API supports CORS, so front-end JavaScript can call it directly.
Yes. The The Calendar API is served over HTTPS, so it's safe to use in production.