I personally manage a site that lists the public holidays of every country of the world, both with long-term forecasts (some of our clients license more than 20 years into the future), and quasi real-time updates, to react when a country makes a last minute change (on average there is one or two such changes, somewhere in the world, everyday).
I won't mention which site it is, to avoid being spammed-out. In any case, which site it is is not relevant to what I am about to write.
All the forecasts are handled automatically by a library of code I have developed since 1989. I have also automated the search for last-minute updates. Yet, despite these 27 years of experience and automated code, and my ability to read news in 5 languages fluently, it takes me 4-5 hours per day, 7 days a week, 365 days a year, to make sure that I miss none of the daily and yearly changes to holidays and their rules, that occur all over the world.
My point ?
You will never find a reliable source of worldwide public holidays that is free. You'll find a few free sites that list the 10-15 "easy" countries. You'll find a few free sites that list more countries, but do not provide dates for non-western calendars. You'll mainly find free sites with lots of ads and cookies.
In some areas, there is still no such thing as a free lunch ;-)