/// <summary>
        /// 取得某月的第一天
        /// </summary>
        /// <param name="datetime">要取得月份第一天的時間</param>
        /// <returns></returns>
        private DateTime FirstDayOfMonth(DateTime datetime)
        {
            return datetime.AddDays(1 - datetime.Day);
        }

        /**//// <summary>
        /// 取得某月的最後一天
        /// </summary>
        /// <param name="datetime">要取得月份最後一天的時間</param>
        /// <returns></returns>
        private DateTime LastDayOfMonth(DateTime datetime)
        {
            return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);
        }

        /**//// <summary>
        /// 取得上個月第一天
        /// </summary>
        /// <param name="datetime">要取得上個月第一天的當前時間</param>
        /// <returns></returns>
        private DateTime FirstDayOfPreviousMonth(DateTime datetime)
        {
            return datetime.AddDays(1 - datetime.Day).AddMonths(-1);
        }

        /**//// <summary>
        /// 取得上個月的最後一天
        /// </summary>
        /// <param name="datetime">要取得上個月最後一天的當前時間</param>
        /// <returns></returns>
        private DateTime LastDayOfPrdviousMonth(DateTime datetime)
        {
            return datetime.AddDays(1 - datetime.Day).AddDays(-1);
        }

 =================================================

.Net中獲取一週第一天、最後一天，一月第一天、最後一天 

DateTime dt = DateTime.Now;
        //本月第一天時間   
        DateTime dt_First = dt.AddDays(-(dt.Day) + 1);
        Label1.Text = dt_First.ToString("yyyy-MM-dd");
        //將本月月數+1 
        DateTime dt2 = dt.AddMonths(1);
        //本月最後一天時間 
        DateTime dt_Last = dt2.AddDays(-(dt.Day));
        Label2.Text = dt_Last.ToString("yyyy-MM-dd");
        int weeknow = Convert.ToInt32(System.DateTime.Now.DayOfWeek);
        int daydiff = (-1) * weeknow + 1;
        int dayadd = 7 - weeknow;

　　 //本週第一天
        lblBegin.Text = System.DateTime.Now.AddDays(daydiff).ToString("yyyy-MM-dd");

       //本週最後一天
        lblEnd.Text = System.DateTime.Now.AddDays(dayadd).ToString("yyyy-MM-dd");