第ｎ週の日付を取得する

        /// <summary>
        /// 第ｎ週の日付を取得する
        /// </summary>
        /// <param name="year">年</param>
        /// <param name="month">月</param>
        /// <param name="n">第ｎ週</param>
        /// <param name="dayOfWeek">曜日の指定</param>
        /// <returns></returns>
        private DateTime NthWDay(int year, int month, int n, DayOfWeek dayOfWeek) {
            int fw = (int)(new DateTime(year, month, 1)).DayOfWeek;
            int firstDay = ((int)dayOfWeek - fw) + 1;
            if (firstDay <= 0)
                firstDay += 7;
            int day = firstDay + (n - 1) * 7;
            return new DateTime(year, month, day);
        }