WEEKDAY, a C++ code which determines the day of the week corresponding to a given date; for instance, the battle of Hastings, on 14 October 1066 (Julian Calendar!), was a Saturday.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
WEEKDAY is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
CALENDAR_RD, a C++ program which computes the representation of a given date in a number of calendrical systems, by Edward Reingold, Nachum Dershowitz
CALPAK, a C++ library which makes various calendar calculations;
DATES, a dataset directory which contains lists of dates in various calendar systems.
DOOMSDAY, a C++ library which is given the year, month and day of a date, and uses John Conway's doomsday algorithm to determine the corresponding day of the week.
TEST_VALUES, a C++ library which supplies test values of various mathematical functions.