Java - Clase calendar y gregorian calendar, ayuda primer dia del mes

SimpleDateFormat sdf = new SimpleDateFormat("yyyy MMM dd HH:mm:ss");

	Calendar calendar = new GregorianCalendar(2013,1,28,13,24,56);

	int year       = calendar.get(Calendar.YEAR);

	int month      = calendar.get(Calendar.MONTH); // Enero = 0, Diciembre = 11

	int dayOfMonth = calendar.get(Calendar.DAY_OF_MONTH);

	int dayOfWeek  = calendar.get(Calendar.DAY_OF_WEEK);

	int weekOfYear = calendar.get(Calendar.WEEK_OF_YEAR);

	int weekOfMonth= calendar.get(Calendar.WEEK_OF_MONTH);

	int hour       = calendar.get(Calendar.HOUR);        // formato 12 hrs

	int hourOfDay  = calendar.get(Calendar.HOUR_OF_DAY); // formato 24 hrs

	int minute     = calendar.get(Calendar.MINUTE);

	int second     = calendar.get(Calendar.SECOND);

	int millisecond= calendar.get(Calendar.MILLISECOND);

	System.out.println(sdf.format(calendar.getTime()));

	System.out.println("año \t\t: " + year);

	System.out.println("mes \t\t: " + month);

	System.out.println("dia del mes \t: " + dayOfMonth);

	System.out.println("dia de la semana \t: " + dayOfWeek);

	System.out.println("Semana del año\t: " + weekOfYear);

	System.out.println("Semana del mes \t: " + weekOfMonth);

	System.out.println("hora \t\t: " + hour);

	System.out.println("hora del dia \t: " + hourOfDay);

	System.out.println("minuto \t\t: " + minute);

	System.out.println("segundo \t\t: " + second);

	System.out.println("millisegundo \t: " + millisecond);