Business Days in c# (a very fast way)

Hi,

Today I needed a simple code that returns the business days between two dates and ended with this:

public static int GetFullWorkingDaysBetween(DateTime firstDate, DateTime lastDate, IEnumerable holidays)
        {
            if (firstDate > lastDate)// Swap the dates if firstDate > lastDate
            {
                DateTime tempDate = firstDate;
                firstDate = lastDate;
                lastDate = tempDate;
            }
            int days = (int)(lastDate.Subtract(firstDate).Ticks / TimeSpan.TicksPerDay);
            int weekReminder = days % 7;
            if (weekReminder > 0)
            {
                switch (firstDate.DayOfWeek)
                {
                    case DayOfWeek.Monday:
                        days = days - ((weekReminder > 5) ? 1 : 0);
                        // Another way for this:
                        //days = days - ((int)weekReminder % 5);
                        // but i think its more expensive
                        break;
                    case DayOfWeek.Tuesday:
                        days = days - ((weekReminder > 4) ? 1 : 0) - ((weekReminder > 5) ? 1 : 0);
                        // The same from above
                        //days = days - ((int)weekReminder % 4);
                        break;
                    case DayOfWeek.Wednesday:
                        days = days - ((weekReminder > 3) ? 1 : 0) - ((weekReminder > 4) ? 1 : 0);
                        break;
                    case DayOfWeek.Thursday:
                        days = days - ((weekReminder > 2) ? 1 : 0) - ((weekReminder > 3) ? 1 : 0);
                        break;
                    case DayOfWeek.Friday:
                        days = days - ((weekReminder > 1) ? 1 : 0) - ((weekReminder > 2) ? 1 : 0);
                        break;
                    case DayOfWeek.Saturday:
                        days = days - 1 - ((weekReminder > 1) ? 1 : 0);
                        break;
                    case DayOfWeek.Sunday:
                        days = days - 1;
                        break;
                }
            }
            days = days - (2 * ((int)days / 7));
            if (holidays != null && holidays.Count() > 0)
            {
                foreach (DateTime holiday in holidays.Where(h => h >= firstDate && h < lastDate))
                {
                    DayOfWeek dayOfWeekOfHoliday = holiday.DayOfWeek;
                    if (dayOfWeekOfHoliday != DayOfWeek.Saturday && dayOfWeekOfHoliday != DayOfWeek.Sunday)
                    {
                        days = days - 1;
                    }
                }
            }
            return days;
        }