C Program to find the day(of week) of birth from a given Date of Birth.

day_of_birth.c

#include<stdio.h>
#include<stdlib.h>

int main() {
	int d, m, y, year, month, day, i;
	
	printf("Enter date of birth (DD MM YYYY) :");
	scanf("%d %d %d", &d, &m, &y);
	if( (d > 31) || (m > 12) || (y < 1900 || y >= 2100) )
	{
		printf("INVALID INPUT. Please enter a valid date between 1900 and 2100");
		exit(0);
	}

	year = y-1900;
	year = year/4;
	year = year+y-1900;
	
	switch(m)
	{
	case 1:
	case 10:
		month = 1;
		break;
	case 2:
	case 3:
	case 11:
		month = 4;
		break;
	case 7:
	case 4:
		month = 0;
		break;
	case 5:
		month = 2;
		break;
	case 6:
		month = 5;
		break;
	case 8:
		month = 3;
		break;
	case 9:
	case 12:
		month = 6;
		break;
	}
	
	year = year + month;
	year = year + d;
	/* Need to make sure extra day is not needed in leap year for dates before March */
	if(( y > 1900 ) && ( y % 4 == 0 ) && ( m < 2 ) )
	year--;
	day  = year % 7;
	
	switch(day)
	{
	case 0:
		printf("Day is SATURDAY\n");
		break;
	case 1:
		printf("Day is SUNDAY\n");
		break;
	case 2:
		printf("Day is MONDAY\n");
		break;
	case 3:
		printf("Day is TUESDAY\n");
		break;
	case 4:
		printf("Day is WEDNESDAY\n");
		break;
	case 5:
		printf("Day is THURSDAY\n");
		break;
	case 6:
		printf("Day is FRIDAY\n");
		break;
	}
	
	return 0;
}