Public holiday API service for several supported countries.
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