Solving without using Calendar class . Solving using logic. Logic Explaination https://www.youtube.com/watch?v=wGZ1uPj6m7Q

    public static String findDay(int month, int day, int year) {
        
        String[] days = {"MONDAY" ,"TUESDAY","WEDNESDAY", "THURSDAY","FRIDAY","SATURDAY","SUNDAY"};
        int[] months = {6,2,2,5,0,3,5,1,4,6,2,4};
        month--;
        
        int temp = year - 2000;
        int rem;
        
        if (temp < 7){    
            rem =  temp + months[month] + day -1;   
            rem %=7;
          
            return days[rem];
        }
        
        temp *= 1.25;
        rem = temp%7;
        
        if(temp % 4 == 0)    temp--;
         
        rem = rem + months[month] + day - 1;   
        rem %=7;
            
        return days[rem];
        
    }

Solving without using Calendar class . Solving using logic

    public static String findDay(int month, int day, int year) {
        
        String[] days = {"MONDAY" ,"TUESDAY","WEDNESDAY", "THURSDAY","FRIDAY","SATURDAY","SUNDAY"};
        int[] months = {6,2,2,5,0,3,5,1,4,6,2,4};
        month--;
        
        int temp = year - 2000;
        int rem;
        
        if (temp < 7){    
            rem =  temp + months[month] + day -1;   
            rem %=7;
          
            return days[rem];
        }
        
        temp *= 1.25;
        rem = temp%7;
        
        if(temp % 4 == 0)    temp--;
         
        rem = rem + months[month] + day - 1;   
        rem %=7;
            
        return days[rem];
        
    }

import java.util.*;

public class Solution {

public static String findDay(int month, int day, int year) {
    Calendar calendar = Calendar.getInstance();
    calendar.set(year, month - 1, day); // Month is 0-based in Calendar
    String[] days = { "SUNDAY", "MONDAY", "TUESDAY", "WEDNESDAY", 
                      "THURSDAY", "FRIDAY", "SATURDAY" };
    int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK);
    return days[dayOfWeek - 1];
}

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    int month = scanner.nextInt();
    int day = scanner.nextInt();
    int year = scanner.nextInt();
    scanner.close();

    String result = findDay(month, day, year);
    System.out.println(result);
}

}

import java.io.; import java.math.; import java.security.; import java.text.; import java.util.; import java.util.concurrent.; import java.util.function.; import java.util.regex.; import java.util.stream.*; import static java.util.stream.Collectors.joining; import static java.util.stream.Collectors.toList;

class Result { public static String findDay(int month,int day,int year) { Calendar c=Calendar.getInstance(); String[] days={"SUNDAY","MONDAY","TUESDAY","WEDNESDAY","THURSDAY","FRIDAY","SATURDAY"}; c.set(year,month-1,day); return days[c.get(Calendar.DAY_OF_WEEK)-1]; } }

public class Sound { public static void main(String[] args) throws IOException { BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in)); BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

    String[] firstMultipleInput = bufferedReader.readLine().replaceAll("\\s+$", "").split(" ");

    int month = Integer.parseInt(firstMultipleInput[0]);

    int day = Integer.parseInt(firstMultipleInput[1]);

    int year = Integer.parseInt(firstMultipleInput[2]);

    String res = Result.findDay(month, day, year);

    bufferedWriter.write(res);
    bufferedWriter.newLine();

    bufferedReader.close();
    bufferedWriter.close();
}

}

i had no idea how to solve this .but thanks to this discussion board.i solved it