Calculate next working day

I have tried to tackle this question as follows:

The program Calculates the next Working day, taking into account:  
   - weekends (Saturdays and Sundays)  
   - holidays happening in the middle of the week 

param name="OrderDate" the date on which the order is placed
param name="workingDays" the number of workingdays to process the order      
returns the date on which the Order is Requested to Ship

```
public static DateTime getNextWorkingDay(DateTime OrderDate, int workingDays)
{
DateTime Holiday = new DateTime();
List SelectedHolidays = new List();
List HolidayList = new List();

int calculatedWorkingDays=0;
if (workingDays OrderDate.Date)
{
int K = (int)(Holiday.Date - OrderDate.Date).TotalDays;
if (K <= calculatedWorkingDays)
{
calculatedWorkingDays++;
}
}
}
}
if (OrderDate.DayOfWeek == DayOfWeek.Sunday)
{
calculatedWorkingDays++;
}
else if (OrderDate.DayOfWeek == DayOfWeek.Saturday || OrderDate.DayOfWeek == DayOfWeek.Friday)
{
calculatedWorkingDays += 2;
}

// this is required date
DateTime date_OrderRequestedToShip = OrderDate.AddDays(calculatedWorkingDays);

//check if the last date fall on a non working day(holiday or weekend)
calculatedWorkingDays = 0;
foreach (DateTime d in SelectedHolidays)
if (d.Date == date_OrderRequestedToShip.Date)
calculatedWorkingDays++;
date_OrderRequestedToShip = date_OrderRequestedToShip.AddDays(calculatedWorkingDays);
if (date_OrderRequestedToShip.DayOfWeek == DayOfWeek.Sunday)
{
calculatedWorkingDays = 1;
}
else if (date_OrderRequestedToShip.DayOfWeek == DayOfWeek.Saturday)
{
calculatedWorkingDays = 2;

First, you should separate a process of parsing your input from your business logic. So, your first step is to convert your holidays input to a structure you'll be working with. So, let's say you have a List where

public struct Holiday
{
    public readonly int Month;
    public readonly int Day;

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

Now, in your place I'd start with implementation of something that enumerates working days like so:

static IEnumerable GetWorkingDays(DateTime startDate, List 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;
        }
    }
}

Note that this iterator is endless, so you should be cautious enumerating it.

Now, your problem can be solved with just one line

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

Complete example here: https://dotnetfiddle.net/W3dalr

StackExchange Code Review Q#83668, answer score: 7