using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public void Main()

    {

        var holidays = new List<Holiday> { new Holiday(3, 13) };

        var orderDate = new DateTime(2015, 3, 9);

        var workingDays = 7;

        var result = GetWorkingDays(orderDate, holidays).Skip(workingDays).First();

        result.Dump();

    }

    static IEnumerable<DateTime> GetWorkingDays(DateTime startDate, List<Holiday> holidays)

    {

        var date = startDate;

        for (;;date = date.AddDays(1))

        {        

            if (date.DayOfWeek != DayOfWeek.Saturday &&

                date.DayOfWeek != DayOfWeek.Sunday &&

                holidays.All(holiday => holiday.Day != date.Day ||

                                        holiday.Month != date.Month))

            {

                yield return date;

            }

        }

    }

}

​

public struct Holiday

{

    public readonly int Month;

    public readonly int Day;        

    public Holiday(int month, int day) { Month = month; Day = day; }

}