Program will accept Year,Month and Date from the user and will display the day of the month.

#include<stdio.h>
#include<conio.h>
#include<math.h>

int isdatevalid(int month, int day, int year)
{
if (day <= 0) return 0 ;
switch( month )
{
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12: if (day > 31) return 0 ; else return 1 ;
case 4:
case 6:
case 9:
case 11: if (day > 30) return 0 ; else return 1 ;
case 2:
if ( day > 29 ) return 0 ;
if ( day < 29 ) return 1 ;

else return 0 ;
}
return 0 ;
}
//------------------------------------------------
int fm(int date, int month,int year)
{
int fmonth,leap;

//leap function 1 for leap & 0 for non-leap

if((year%100==0) && (year%400!=0))
leap=0;
else if(year%4==0)
leap=1;
else
leap=0;

fmonth=3+(2-leap)*((month+2)/(2*month))+(5*month+month/9)/2;
//f(m) formula

fmonth = fmonth % 7; //bring it in range of 0 to 6

return fmonth;
}

//----------------------------------------------
int day_of_week(int date, int month, int year)
{
int dow; //day of week

int YY = year % 100;
int century = year / 100;

printf("nDate: %d/%d/%dnn",date,month,year);

dow = 1.25 *  YY + fm(date,month,year) + date - 2*( century % 4);
//function of weekday for Gregorian

dow = dow % 7; //remainder on division by 7

switch (dow)
{
case 0:
printf("weekday = Saturday");
break;
case 1:
printf("weekday = Sunday");
break;
case 2:
printf("weekday = Monday");
break;
case 3:
printf("weekday = Tuesday");
break;
case 4:
printf("weekday = Wednesday");
break;
case 5:
printf("weekday = Thursday");
break;
case 6:
printf("weekday = Friday");
break;
default:
printf("Incorrect data");
}
return 0;
}
//------------------------------------------
void main()
{
int date,month,year;
clrscr();

printf("Enter the year ");
scanf("%d",&year);

printf("Enter the month ");
scanf("%d",&month);

printf("Enter the date ");
scanf("%d",&date);

day_of_week(date,month,year);

getch();
}

Output :

Enter the year 2012
Enter the month 02
Enter the date 29

Date: 29/2/2012

weekday = Wednesday