\Finds the day of the week of a date using John Conway's Doomsday rule.
\Returns the day of the week (Sunday = 0, Monday = 1,...) for the date
\ specified by CCYY, MM and DD.
function DOW ( CCYY, MM, DD );
integer CCYY, MM, DD;
integer Doomsday, AnchorDay, LeapYear, Dooms;
begin
Doomsday  := rem(( \Tuesday \2
                 + 5 * rem( CCYY/4 )
                 + 4 * rem( CCYY/100 )
                 + 6 * rem( CCYY/400 )
                 ) / 7);
LeapYear  := rem(CCYY/4) = 0 and ( rem(CCYY/100) # 0 or rem(CCYY/ 400) = 0 );
Dooms     := [[4, 1, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5],
              [3, 7, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5]];
AnchorDay := Dooms(if LeapYear then 0 else 1, MM-1);
return rem( ( Doomsday + ( DD - AnchorDay ) + 7 ) / 7);
end; \DOW

\Prints a test date and its day of the week
procedure TestDOW ( CCYY, MM, DD );
integer CCYY, MM, DD;
integer DayName;
begin
DayName := ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday",
            "Saturday"];
IntOut(0, CCYY);
ChOut(0, ^-);  IntOut(0, MM/10);  IntOut(0, rem(0));
ChOut(0, ^-);  IntOut(0, DD/10);  IntOut(0, rem(0));
Text(0, ": "); Text(0, DayName( DOW( CCYY, MM, DD ) ));
CrLf(0);
end;

begin   \task test cases
TestDOW( 1800,  1,  6 );
TestDOW( 1875,  3, 29 );
TestDOW( 1915, 12,  7 );
TestDOW( 1970, 12, 23 );
TestDOW( 2043,  5, 14 );
TestDOW( 2077,  2, 12 );
TestDOW( 2101,  4,  2 );
TestDOW( 2022,  6, 19 )
end