Javaは1つの日付が何曜日かを判断します

public static String getWeekOfDate(String day, SimpleDateFormat dt) {
		Date date = null;
		try {
			date = dt.parse(day);
		} catch (ParseException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		if (date == null) {
			return "";
		}
		String[] weekDays = { "   ", "   ", "   ", "   ", "   ", "   ", "   " };
		Calendar cal = Calendar.getInstance();
		cal.setTime(date);
		int w = cal.get(Calendar.DAY_OF_WEEK) - 1;
		if (w < 0) {
			w = 0;
		}
		return weekDays[w];
	}