注意，是該月中的周次，一週最多有6周。

        <span style="background-color: rgb(102, 51, 255);">//獲取周開始日期和結束日期（週日-週六）
        public string GetWeekDate(int yearly, int monthly, string weekly)
        {
            string dayStr="";

            try
            {
                DateTime day1 = DateTime.Parse(yearly.ToString() + "-" + monthly.ToString() + "-01");//該月第一天日期
                int weekOfDay1 = (Int32)day1.DayOfWeek;//該月第一天是周幾
                int dayCount = DateTime.DaysInMonth(yearly, monthly);//該月天數

                int endDay = 0;//結束日
                DateTime dateEnd = DateTime.Now;//周結束日期,初始化為當前時間

                //7*周次 - 第一週中上月天數 > 本月天數
                if (7 * Int32.Parse(weekly) - weekOfDay1 > dayCount)
                {
                    endDay = 7 * Int32.Parse(weekly) - weekOfDay1 - dayCount;
                    DateTime dateLast = DateTime.Parse(yearly.ToString() + "-" + monthly.ToString() + "-" + dayCount.ToString());//該月最後一天日期
                    dateEnd = dateLast.AddDays(endDay);
                }
                //7*周次 - 第一週中上月天數 > 本月天數
                if (7 * Int32.Parse(weekly) - weekOfDay1 <= dayCount)
                {
                    endDay = 7 * Int32.Parse(weekly) - weekOfDay1;
                    dateEnd = DateTime.Parse(yearly.ToString() + "-" + monthly.ToString() + "-" + endDay.ToString());
                }

                DateTime dateStart = dateEnd.AddDays(-6); //周開始日期

                dayStr = dateStart.ToString("yyyy-MM-dd") +" ~ " + dateEnd.ToString("yyyy-MM-dd");
            }
            catch (Exception)
            {
                throw;
            }
            return dayStr;
        }</span>
 

呼叫：var resultWeekDay = GetWeekDate(2015, 4, 5);//4月第5周

結果：2015-04-26  ~ 2015-05-02