Calculate the number of business days between two dates

BusinessDaysUntil.cs

/// <summary>
/// Calculates number of business days, taking into account:
///  - weekends (Saturdays and Sundays)
///  - bank holidays in the middle of the week
/// </summary>
/// <param name="firstDay">First day in the time interval</param>
/// <param name="lastDay">Last day in the time interval</param>
/// <param name="bankHolidays">List of bank holidays excluding weekends</param>
/// <returns>Number of business days during the 'span'</returns>
public static int BusinessDaysUntil(this DateTime firstDay, DateTime lastDay, params DateTime[] bankHolidays)
{
    firstDay = firstDay.Date;
    lastDay = lastDay.Date;
    if (firstDay > lastDay)
        throw new ArgumentException("Incorrect last day " + lastDay);

    TimeSpan span = lastDay - firstDay;
    int businessDays = span.Days + 1;
    int fullWeekCount = businessDays / 7;
    // find out if there are weekends during the time exceedng the full weeks
    if (businessDays > fullWeekCount*7)
    {
        // we are here to find out if there is a 1-day or 2-days weekend
        // in the time interval remaining after subtracting the complete weeks
        int firstDayOfWeek = (int) firstDay.DayOfWeek;
        int lastDayOfWeek = (int) lastDay.DayOfWeek;
        if (lastDayOfWeek < firstDayOfWeek)
            lastDayOfWeek += 7;
        if (firstDayOfWeek <= 6)
        {
            if (lastDayOfWeek >= 7)// Both Saturday and Sunday are in the remaining time interval
                businessDays -= 2;
            else if (lastDayOfWeek >= 6)// Only Saturday is in the remaining time interval
                businessDays -= 1;
        }
        else if (firstDayOfWeek <= 7 && lastDayOfWeek >= 7)// Only Sunday is in the remaining time interval
            businessDays -= 1;
    }

    // subtract the weekends during the full weeks in the interval
    businessDays -= fullWeekCount + fullWeekCount;

    // subtract the number of bank holidays during the time interval
    foreach (DateTime bankHoliday in bankHolidays)
    {
        DateTime bh = bankHoliday.Date;
        if (firstDay <= bh && bh <= lastDay)
            --businessDays;
    }

    return businessDays;
}

Originally from: http://stackoverflow.com/a/1619375/197634