Church Calendar
Catholic liturgical calendar
Dates and timezone-correct countdowns for 150+ holidays and observances in 18 countries.
CountdownsHQ 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.
A free, public JSON API for holidays, dates and time-zone-accurate countdowns. No key required. Endpoints for holidays, countries and live countdowns.
Details from countdownshq.com.

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Catholic liturgical calendar
Lookup for a name and returns nameday date
Industry-leading holiday data with over 5,000 holidays and thousands of descriptions
Fastest and most advanced public holiday and observance service on the market
Worldwide Holidays
The CountdownsHQ API is provided by CountdownsHQ (countdownshq.com). A free, public JSON API for holidays, dates and time-zone-accurate countdowns. No key required. Endpoints for holidays, countries and live countdowns.
Yes. The CountdownsHQ 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 CountdownsHQ API requires no authentication, so you can start making requests without signing up.
Yes. The CountdownsHQ API supports CORS, so front-end JavaScript can call it directly.
Yes. The CountdownsHQ API is served over HTTPS, so it's safe to use in production.