## Zeller's Congruence

In this lesson we are going to figure out what day of the week it is after being given any given set of dates.

Have you ever wondered how to find a weekday for any (past or future) date?

Okay, you can check it in a calendar (you probably have one on your smartphone), but this is no solution for a coder.

One of the most popular algorithms for this task is the so-called "Zeller's congruence".

Sounds complicated?

Nothing could be further from the truth, and we're going to show you exactly that.

You'll need three values:

year number (int – let's assume that we're interested only in dates from the 20th and 21st centuries);

month number (int – 1 to 12);

day number (int – 1 to it depends)

Be patient – this will take a while:

1. Decrease month number by 2;

2. if month number becomes less than 0, increment it by 12 and decrement year by 1;

3. take month number and multiply it by 83 and divide it by 32;

4. add day number to month;

5. add year number to month;

6. add year/4 to month;

7. subtract year/100 from month;

8. add year/400 to month;

9. find the remainder of dividing month by 7;

10. Congrats! A weekday number is ready for you! 0 – Sunday, 1 – Monday, ... and so on.

We want you to write a code which finds a weekday number for a date entered by a user.

The program should ask the user for the year, month and day (in this order) and output a value indicating a weekday. Make your code as smart as possible. Test your code using the data we've provided.

Have you ever wondered how to find a weekday for any (past or future) date?

Okay, you can check it in a calendar (you probably have one on your smartphone), but this is no solution for a coder.

One of the most popular algorithms for this task is the so-called "Zeller's congruence".

Sounds complicated?

Nothing could be further from the truth, and we're going to show you exactly that.

You'll need three values:

year number (int – let's assume that we're interested only in dates from the 20th and 21st centuries);

month number (int – 1 to 12);

day number (int – 1 to it depends)

Be patient – this will take a while:

1. Decrease month number by 2;

2. if month number becomes less than 0, increment it by 12 and decrement year by 1;

3. take month number and multiply it by 83 and divide it by 32;

4. add day number to month;

5. add year number to month;

6. add year/4 to month;

7. subtract year/100 from month;

8. add year/400 to month;

9. find the remainder of dividing month by 7;

10. Congrats! A weekday number is ready for you! 0 – Sunday, 1 – Monday, ... and so on.

We want you to write a code which finds a weekday number for a date entered by a user.

The program should ask the user for the year, month and day (in this order) and output a value indicating a weekday. Make your code as smart as possible. Test your code using the data we've provided.

Sample code

None for this lesson.....

Assignment

Build the program described above and get it to work correctly. To make sure that it is working correctly here are some dates that you can use to test it against.

Example input

2016

2

10

Example output

3

Example input

2000

1

1

Example output

6

Example input

1999

12

31

Example output

5

Example input

1964

12

21

Example output

1

Example input

2016

2

10

Example output

3

Example input

2000

1

1

Example output

6

Example input

1999

12

31

Example output

5

Example input

1964

12

21

Example output

1

## EXTRA CREDIT!!!

Instead of outputting some stupid number, like 0-6, write an extra line or two of code to have it print out the actual day of the week.

So.... understanding that;

0 = Sunday

1 = Monday

2 = Tuesday

3 = Wednesday

4 = Thursday

5 = Friday

6 = Saturday

Have the code print out Monday instead of 0 and so on.

So.... understanding that;

0 = Sunday

1 = Monday

2 = Tuesday

3 = Wednesday

4 = Thursday

5 = Friday

6 = Saturday

Have the code print out Monday instead of 0 and so on.