import java.util.; import java.io.;

public class DayOfWeekFinder {

public static String findDay(int day, int month, int year) {
    Calendar calendar = Calendar.getInstance();
    calendar.set(year, month - 1, day);
    return calendar.getDisplayName(Calendar.DAY_OF_WEEK, Calendar.LONG, Locale.getDefault()).toUpperCase();
}

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

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

}

public static String findDay(int month, int day, int year) {
    LocalDate date = LocalDate.of(year, month, day);
    return date.getDayOfWeek().toString();
}

I couldn't figure out how to solve the problem using the Calender class.

So I used LocalDateTime and DateTimeFormatter classes instead. This approach appears clearer to me also. Note :- You would also need to import java.time.LocalDateTime and java.time.format.DateTimeFormatter on the toplevel of the file. This is the code of the method -

public static String findDay(int month, int day, int year) {
        LocalDateTime date = LocalDateTime.of(year, month, day,0,0);
        
        DateTimeFormatter dateFormat = DateTimeFormatter.ofPattern("EEEE");
        
        String formattedDate = date.format(dateFormat).toString();
        
        String DAY = formattedDate.toUpperCase();
        return DAY;
    }

/* * Complete the 'findDay' function below. * * The function is expected to return a STRING. * The function accepts following parameters: * 1. INTEGER month * 2. INTEGER day * 3. INTEGER year */

public static String findDay(int month, int day, int year) {
    Calendar g = new GregorianCalendar();
    g.set(Calendar.YEAR, year);
    g.set(Calendar.MONTH, month-1);
    g.set(Calendar.DAY_OF_MONTH, day);
    return g.getDisplayName(Calendar.DAY_OF_WEEK, Calendar.LONG, Locale.getDefault()).toUpperCase();
}