Best Way to Code

First Method

        public static int GetWorkingDays(DateTime from, DateTime to)
        {
            var workingDay = 1 + (to - from).Days;
            return Enumerable
                .Range(0, workingDay)
                .Select(x => from.AddDays(x))
                .Count(x => x.DayOfWeek != DayOfWeek.Saturday && x.DayOfWeek != DayOfWeek.Sunday);
        }

Second Method

        public static double GetWorkingDays(DateTime startD, DateTime endD)
        {
            double workingDay = 1 + ((endD - startD).TotalDays * 5 - (startD.DayOfWeek - endD.DayOfWeek) * 2) / 7;

            if (endD.DayOfWeek == DayOfWeek.Saturday) workingDay--;
            if (startD.DayOfWeek == DayOfWeek.Sunday) workingDay--;

            return workingDay;
        }