How to add Holiday Calendars...

See this article Birthdays, holidays, and other calendars - Computer - Calendar Help

You have to scroll down and click on drop down arrows of regional arrow , scroll down to the end when you find holidays in Thailand check the box of it , then it will be added in your gmail calendar and simultaneously also in apple calendar .

An example using gmail service -

Sign in gmail calendar by entering gmail address and password , go to settings and check the box for christian holidays you can add the desired options also , they will be added in your calendar application also .

Subscribe to calendars on Mac - Apple Support

Use iCloud calendar subscriptions - Apple Support

See if anything here helps.

https://www.calendarlabs.com/ical-calendar//