Festivo Public Holidays
Fastest and most advanced public holiday and observance service on the market

Display, create and modify Google calendar events.
Google Calendar API is a free Calendar API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
"Discover resources about enhancing the Calendar experience, automating tasks, or integrating your service."
Details from developers.google.com.

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Fastest and most advanced public holiday and observance service on the market
Lookup for a name and returns nameday date
The Google Calendar API is provided by Google Calendar (developers.google.com). "Discover resources about enhancing the Calendar experience, automating tasks, or integrating your service."
Yes. The Google 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.
Yes. The Google Calendar API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the Google Calendar API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Google Calendar API is served over HTTPS, so it's safe to use in production.