# Formula for calculating the day of week from a given date

Updated on February 23, 2015

## The formula for calculating the day is-

day=(y + j + a - b + ab) %7

j = julian day of the date

y = year of given date (in 4 digits)

a = int part of (y-1)/4

b = int part of (y-1)/100

ab=int part of (y-1)/400

## The value of variable day tells us the day of week

Value of variable day
Name of day of week
0
Saturday
1
Sunday
2
Monday
3
Tuesday
4
Wednesday
5
Thursday
6
Friday

## In the above formula, julian day of a date represents the day of year.

Julian day of:

1st Jan = 1

5th Feb = 36

31st Dec = 365 (366 if leap year)

## Code (In 'C' Language)

```#include<stdio.h>
#include<conio.h>
void main()
{
int d,m,y,j,a,b,ab,day;
printf("Enter date (dd/mm/yyyy) : ");
scanf("%d/%d/%d",&d,&m,&y);
j=d;
switch(m-1)
{
case 11:
j+=30;
case 10:
j+=31;
case 9:
j+=30;
case 8:
j+=31;
case 7:
j+=31;
case 6:
j+=30;
case 5:
j+=31;
case 4:
j+=30;
case 3:
j+=31;
case 2:
j+=28;
case 1:
j+=31;
}
if(y%4==0 && y%100 !=0 || y%400==0)
if(m!=1&&m!=2)
j=j+1;

a=(y-1)/4;
b=(y-1)/100;
ab=(y-1)/400;
day=(y+j+a-b+ab)%7;
switch(day)
{
case 0:
printf("Saturday\n");
break;
case 1:
printf("Sunday\n");
break;
case 2:
printf("Monday\n");
break;
case 3:
printf("Tuesday\n");
break;
case 4:
printf("Wednesday\n");
break;
case 5:
printf("Thursday\n");
break;
case 6:
printf("Friday\n");
break;
}
}
```

## Program to find the LCM and HCF of two numbers

```#include<stdio.h>
#include<conio.h>
void main()
{
int a,b,c,d;
printf("Enter 1st Number: ");
scanf("%d",&a);
printf("Enter 2nd Number: ");
scanf("%d",&b);
c=a;
d=b;
while(c!=d)
{
if(c<d)
c=c+a;
else
d=d+b;
}
printf("LCM of %d and %d is %d\n",a,b,c);
c=a;
d=b;
while(c!=d)
{
if(c>d)
c=c-d;
else
d=d-c;
}
printf("HCF of %d and %d is %d\n",a,b,c);
}```

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