#include <stdio.h>

main()

//----------------------------------------------------------
// Function to validate date format
// returns "0" if format is invalid,
// otherwise returns "1"

int isValidDate(int day, int month, int year)

{
 if ((day < 1) || (day > 31))
   return 0;
 if ((month < 1) || (month > 12))
   return 0;
 if ((year < 1901) || (year > 2099))
   return 0;
 return 1;
}

//----------------------------------------------------------
// Function to calculate the day of week and define
// day_of_week accordingly (0 = Monday, 6 = Sunday)

int dayOfWeek(int day, int month, int year)

{
 // Calculate passed days of current year:

 int daysPassed = 0;
 int monthsCounted = 1;

 while (monthsCounted < month)
   
 {
   if ((monthsCounted == 1) ||
       (monthsCounted == 3) ||
       (monthsCounted == 5) ||
       (monthsCounted == 7) ||
       (monthsCounted == 8) ||
       (monthsCounted == 10) ||
       (monthsCounted == 12))

       {
         daysPassed = daysPassed + 31;
       }

   if ((monthsCounted == 4) ||
       (monthsCounted == 6) ||
       (monthsCounted == 9) ||
       (monthsCounted == 11))

       {
         daysPassed = daysPassed + 30;
       }

   if (monthsCounted == 2)

      {
        daysPassed = daysPassed + 28;
      }

   monthsCounted++;

 }

 daysPassed = daysPassed + day;

 // add days for passed years

 daysPassed = daysPassed + ((year-1901)*365);


 // consider leap years

 int leap years = (year - 1901) /  4;

 daysPassed = daysPassed + leap years;


 // if current year is a leap year, add one day to passed days

 if ((year % 4) == 0)

 daysPassed++;

 return (daysPassed % 7);

}


//----------------------------------------------------------
// Function to print weekday in English depending
// on day_of_week

void printWeekDay(int day_of_week)

{
 switch (day_of_week)

 {
   case 0 : printf("Monday\n");
            break;
   case 1 : printf("Tuesday\n");
            break;
   case 2 : printf("Wednesday\n");
            break;
   case 3 : printf("Thursday\n");
            break;
   case 4 : printf("Friday\n");
            break;
   case 5 : printf("Saturday\n");
            break;
   case 6 : printf("Sunday\n");
            break; 
 }
}


//----------------------------------------------------------
//Main program
//
// Gets input date and calls previously defined
// functions to calculate and output weekday ov given date
// Will return "0" if input date is invalid,
// otherwise will output weekday in english and return "1"

int main ()

{
 int day = 0, month = 0, year = 0;

 printf("Enter DD/MM/YYYY\n");
 scanf("%02d/%02d/%04d", &day, &month, &year);

 if (isValidDate(day, month, year) == 0)
 {
  printf("Invalid date: %02d/%02d/%04d", day, month, year);
  return 1;
 }

 int day_of_week = dayOfWeek(day, month, year);
 printWeekDay(day_of_week);
 return 0;
}