Chapter 3 Exercise 21, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

**3.21 (Science: day of the week) Zeller’s congruence is an algorithm developed by Christian Zeller to calculate the day of the week. The formula is $h=(q+26\frac{m+1}{10}+k+\frac{k}{4}+\frac{j}{4}+5j)\%7$
where ■ h is the day of the week (0: Saturday, 1: Sunday, 2: Monday, 3: Tuesday, 4: Wednesday, 5: Thursday, 6: Friday). ■ q is the day of the month. ■ m is the month (3: March, 4: April, …, 12: December). January and February are counted as months 13 and 14 of the previous year. ■ j is the century (i.e., year 100 ). ■ k is the year of the century (i.e., year % 100). Note that the division in the formula performs an integer division. Write a program that prompts the user to enter a year, month, and day of the month, and displays the name of the day of the week. Here are some sample runs:

Enter year: (e.g., 2012): 2015
Enter month: 1-12: 1
Enter the day of the month: 1-31: 25
Day of the week is Sunday

Enter year: (e.g., 2012): 2012 Enter month: 1-12: 5
Enter the day of the month: 1-31: 12
Day of the week is Saturday

(Hint: January and February are counted as 13 and 14 in the formula, so you need to convert the user input 1 to 13 and 2 to 14 for the month and change the year to the previous year.) 

import java.util.Scanner;
 
public class ProgrammingEx3_21 {
 
 public static void main(String[] args) {
  Scanner input = new Scanner(System.in);
 
  System.out.print("Enter year: (e.g., 2012):");
  int y = input.nextInt();
  System.out.print("Enter month: 1-12:");
  int m = input.nextInt();
  System.out.print("Enter the day of the month: 1-31:");
  int q = input.nextInt();
 
  // dealing with Jan and Feb
  if (m == 1 || m == 2) {
   m = m + 12;
   y = y - 1;
  }
 
  int j = y / 100;
  int k = y % 100;
  int h = (q + 26 * (m + 1) / 10 + k + k / 4 + j / 4 + 5 * j) % 7;
  String day = "";
 
  switch (h) {
 
  case 0:
   day = "Saturday";
   break;
  case 1:
   day = "Sunday";
   break;
  case 2:
   day = "Monday";
   break;
  case 3:
   day = "Tuesday";
   break;
  case 4:
   day = "Wednesday";
   break;
  case 5:
   day = "Thursday";
   break;
  case 6:
   day = "Friday";
   break;
 
  }
 
  System.out.print("Day of the week is " + day);
 
 }
}