AddWorkdays

public static class DateTimeExtensions
{
    public static bool IsWeekend(this DayOfWeek d) 
    { 
        return !d.IsWeekday(); 
    }
 
    public static bool IsWeekday(this DayOfWeek d)
    {
        switch (d)
        {
            case DayOfWeek.Sunday:
            case DayOfWeek.Saturday: return false;
 
            default: return true;
        }
    }
 
    public static DateTime AddWorkdays(this DateTime d, int days)
    {
        // start from a weekday
        while (d.DayOfWeek.IsWeekday()) d = d.AddDays(1.0);
        for (int i = 0; i < days; ++i)
        {
            d = d.AddDays(1.0);
            while (d.DayOfWeek.IsWeekday()) d = d.AddDays(1.0);
        }
        return d;
    }
}

Kullanımı

DateTime due = DateTime.Today.AddWorkdays(10); // due in 10 workdays