using static System.Console;

using System;

public class Program {

    public static void Main()   {

        WriteLine(AddBusinessDays(DateTime.Now, 8));

        WriteLine(AddBusinessDays(new DateTime(2015, 10, 26), 15));

    }

​

    public static DateTime AddBusinessDays(DateTime date, int days) {

        if (days < 0) {

            throw new ArgumentException("days cannot be negative", "days");

        }

        if (days == 0) return date;

        if (date.DayOfWeek == DayOfWeek.Saturday) {

            date = date.AddDays(2);

            days -= 1;

        }

        else if (date.DayOfWeek == DayOfWeek.Sunday) {

            date = date.AddDays(1);

            days -= 1;

        }

        date = date.AddDays(days / 5 * 7);

        int extraDays = days % 5;

        if ((int)date.DayOfWeek + extraDays > 5) {

            extraDays += 2;

        }

        return date.AddDays(extraDays);

    }

}