Assign A Value To Numeric Variable Then Manipulate It And Return A New String

At first I wanted to just use an array and set each day a value, however I was told that it has to be stored as a string.

Design and implement the class Day that implements the day of the week in a program. The class Day should store the day, such as Sun for Sunday. The program should be able to perform the following operations on an object of type Day:

A. Set the day.
B. Print the day.
C. Return the day.
D. Return the next day.
E. Return the previous day.
F. Calculate and return the day by adding certain days to the current day. For example, if the current day is Monday and we add four days, the day to be returned is Friday. Similarly, if today is Tuesday and we add 13 days, the day to be returned is Monday.
G. Add the appropriate constructors.
H. Write the definitions of the methods to implement the operations for the class Day, as defined in A through G.
I. Write a program to test various operations on the class Day.

import java.util.*;
public class Day {
static Scanner readinput = new Scanner(System.in);
String day;
public Day(String day) {

[Code] ....

So right now if I run my code it allows me to type in a day, then it gives me the next and previous day, the last part is to add X days to it. Ideally I would like to be able to take the day entered, depending on the day set a numeric value, then add prompt for number of days you want to add. The program should then use modal (%7 ) to add value to the value that was given based on the day, and then translate it back into a String value (the day).