As of Version 10.0, calendar functionality is built into the Wolfram Language

Calendar Package

 

ReferenceReference

DayOfWeek day of the week for a given date

DaysPlus add a number of days to a given date

DaysBetween number of days between two dates

CalendarChange convert a date from one calendar to another

EasterSunday date of Easter Sunday for a given year

EasterSundayGreekOrthodox Easter Sunday according to the Greek Orthodox Church

JewishNewYear date of the Jewish New Year

DateQ whether a date is valid in the current calendar system

Sunday  ▪  Monday  ▪  Tuesday  ▪  Wednesday  ▪  Thursday  ▪  Friday  ▪  Saturday

Calendars

Gregorian  ▪  Julian  ▪  Islamic  ▪  Jewish