獲取本月的第幾周：

private  static int getWeekNumInMonth(DateTime daytime)
        {
            int dayInMonth = daytime.Day;
            //本月第一天
            DateTime firstDay = daytime.AddDays(1 - daytime.Day);
            //本月第一天是周幾
            int weekday = (int)firstDay.DayOfWeek == 0 ? 7 : (int)firstDay.DayOfWeek;
            //本月第一週有幾天
            int firstWeekEndDay = 7 - (weekday - 1);
            //當前日期和第一週之差
            int diffday = dayInMonth - firstWeekEndDay;
            diffday = diffday > 0 ? diffday : 1;
            //當前是第幾周,如果整除7就減一天
            int WeekNumInMonth = ((diffday % 7) == 0
             ? (diffday / 7 - 1)
             : (diffday / 7)) + 1 + (dayInMonth > firstWeekEndDay ? 1 : 0);
            return WeekNumInMonth;
        }
 

 GregorianCalendar gc = new GregorianCalendar();
            int weekOfYear = gc.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
            Console.WriteLine(weekOfYear);
            Console.ReadLine();