Historical data regarding holidays.
Holidays API is a free Calendar API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Don't let sourcing and maintaining holiday information get in the way of building your product. Start developing for free.
Details from holidayapi.com.

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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.
Fastest and most advanced public holiday and observance service on the market
The Holidays API is provided by Holidays are hard. Holiday API makes them easy. (holidayapi.com). Don't let sourcing and maintaining holiday information get in the way of building your product. Start developing for free.
Yes. The Holidays 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.
Yes. The Holidays API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Holidays API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Holidays API is served over HTTPS, so it's safe to use in production.