Calendarific is a simple way to get all holidays for any country in the world via a JSON API
Developer Almanac for fun
Worldwide Holidays and Working Days
Public holidays for more than 90 countries
Check if a date is a Russian holiday or not
Lookup for a date and returns name
Convert between Gregarian and Hebrew, fetch Shabbat and Holiday times, etc.
What name and when has nameday
Display, create and modify Google calendar events
Free API for obtaining information about holidays.