Calculates the date of Easter in Python 3 according to the Julian, Revised Julian, and Gregorian calendars.
Requires two arguments:
- the year, in which to calculate Easter (in format "YYYY");
- the calendar to use (1 = Julian, 2 = Revised Julian, 3 = Gregorian).
The second argument is optional; if it is not used, the script will default to calculatig Easter according to the Gregorian calendar.
Most Eastern Orthodox Churches use the Revised Julian calendar to calculate Easter, except the Russian Orthodox, Serbian Orthodox, and Mount Athos, which all use the Julian calendar.