Official Indian public holidays for the Central government and 36 states/UTs.
India Public Holidays 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.
Comprehensive developer reference for all 5 Indian Calendar REST endpoints, state filters, range queries, and JSON response models.
Details from calendar-api-web.vercel.app.

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Yes. The India Public 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.
No. The India Public Holidays API requires no authentication, so you can start making requests without signing up.
Yes. The India Public Holidays API supports CORS, so front-end JavaScript can call it directly.
Yes. The India Public Holidays API is served over HTTPS, so it's safe to use in production.