Q: I want code that gives the weekday of the given date.

 A1: There is a 
WKDAYFN function in my Turbo Pascal units collection
ftp://garbo.uwasa.fi/pc/ts/tspa3470.zip (or whatever version 
number
is the latest, and where 70 is 40 50 55 60 and 70) Turbo Pascal
units collection to give the modern weekday based on Zeller's
congruence. Available by anonymous FTP or mail server from
garbo.uwasa.fi. Also you can find a more extensive Julian and
Gregorian weekday algorithm with source code in Dr.Dobb's 
Journal,
June 1989, p. 148. Furthermore Press & Flannery & al 
(1986),
Numerical Recipes, Cambridge University Press, present a 
weekday
code. The Numerical Recipes codes are available as
ftp://garbo.uwasa.fi/pc/turbopas/nrpas13.zip (big, 404k!).

 A2: Some will 
recommend the following kludge. Store the current
date, change it, and let MS-DOS get you the weekday. Don't use it!
It is a bad suggestion. On top of being sloppy programming, there
are several snags.  The trick works only for years 1980-2079. A
crash the program may leave the clock at a wrong date. And even 
if
multitasking is rare, in a multitasking environment havoc may result
for the other tasks. And you may have a TSR that requires the
correct date, etc.