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