Hi! Why does setting the first day of week to Sunday changes week number? This piece of code

    public static void main(String[] args) throws Exception {
        System.out.println(System.getProperty("java.runtime.name") + " " +
                System.getProperty("java.runtime.version"));
        Locale fiFI = new Locale("fi", "FI");
        Calendar cal = new GregorianCalendar(fiFI);
        printWeek(cal);
        cal.setFirstDayOfWeek(Calendar.SUNDAY);
        printWeek(cal);
    }

    private static void printWeek(Calendar cal) {
        System.out.println(cal.getTime() + " -> week " + cal.get(Calendar.WEEK_OF_YEAR));
    }

prints out the following lines:

     Java(TM) SE Runtime Environment 1.8.0_31-b13
     Mon Mar 23 10:48:06 EET 2015 -> week 13
     Mon Mar 23 10:48:06 EET 2015 -> week 12

Best regards,

    Harri