```Contributor: EARL DUNOVANT

{
EARL DUNOVANT

> Which date is what day For a particular month.

Zeller's Congruence is an algorithm that calculates a day of the week given
a year, month and day. Created in 1887(!). Jeff Duntemann of PC Techniques
fame implemented it in TP in the 11/90 issue of Dr Dobbs Journal, With a
(115 min left), (H)elp, More? major kludge because TP's MOD operator returns a remainder instead of a
True mathematical modulus. I added the Kludge Alert banner that I use in my
own code.
}

Function CalcDayOfWeek(Year, Month, Day : Integer) : Integer;
Var
Century,
Holder  : Integer;
begin
{ First test For error conditions on input values: }
if (Year < 0) or (Month < 1) or (Month > 12) or (Day < 1) or (Day > 31) then
CalcDayOfWeek := -1  { Return -1 to indicate an error }
else
{ Do the Zeller's Congruence calculation as Zeller himself }
{ described it in "Acta Mathematica" #7, Stockhold, 1887.  }
begin
{ First we separate out the year and the century figures: }
Century := Year div 100;
Year    := Year MOD 100;
{ Next we adjust the month such that March remains month #3, }
{ but that January and February are months #13 and #14,     }
{ *but of the previous year*: }
if Month < 3 then
begin
Inc(Month, 12);
if Year > 0 then
Dec(Year, 1)      { The year before 2000 is }
else              { 1999, not 20-1...       }
begin
Year := 99;
Dec(Century);
end;
end;

{ Here's Zeller's seminal black magic: }
Holder := Holder + (((Month + 1) * 26) div 10); { Calc the increment }
Holder := Holder + Year;              { Add in the year }
Holder := Holder + (Year div 4);      { Correct For leap years  }
Holder := Holder + (Century div 4);   { Correct For century years }
Holder := Holder - Century - Century; { DON'T KNOW WHY HE DID THIS! }
While Holder < 0 do                   { Get negative values up into }
Inc(Holder, 7);                     { positive territory before   }
{ taking the MOD...         }
Holder := Holder MOD 7;               { Divide by 7 but keep the  }
{ remainder rather than the }
{ quotient }