print (datepart(weekday, '2017-04-24') + 5) % 7
print (datepart(weekday, '2017-04-26') + 5) % 7
print (datepart(weekday, '2017-04-30') + 5) % 7
--周一到周日 0-6

        public static int GetWeekdayNum(DateTime dt) {
            dt = new DateTime(dt.Year, dt.Month, 1);
            var monthDays = dt.Subtract(dt.AddMonths(1));
            int num = 0;
            for(int i = 1; i <= Math.Abs(monthDays.TotalDays) - 28; i++) {
                var dtTemp = new DateTime(dt.Year, dt.Month, 28 + i);
                if(dtTemp.DayOfWeek != DayOfWeek.Saturday && dtTemp.DayOfWeek != DayOfWeek.Sunday) {
                    num++;
                }
            }
            return 20 + num;
        }