DOOMSDAY is 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.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
DOOMSDAY is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version and a Python version.
CALENDAR_NYT, a FORTRAN90 library which shows the correspondence between dates and the New York Times volume and issue number;
CALENDAR_RD, a C++ program which computes the representation of a given date in a number of calendrical systems, by Edward Reingold and Nachum Dershowitz
CALPAK, a C++ library which makes various calendar calculations;
DATES, a dataset directory which contains lists of dates in various calendar systems.
WEEKDAY, a C library which determines the day of the week for a given day.
You can go up one level to the C source codes.