A library to calculate the day of the week using Zellers' Congruence for Julian and Gregorian calendars.
Examples:
2016 April 17 ok
cr cr cr print-gregorian-day-of-week cr cr cr
17 of April 2016 is a Sunday
ok cr cr cr print-julian-day-of-week cr cr cr
17 of April 1500 is a Sunday
ok