UKBankHolidayCalculator.cs

public class BankHolidayCalculator
{
    public bool IsJanuaryBankHolidayMonday(DateTime currentDateTime)
    {
        // January bank holiday falls on the first working day after New Year's day,
        // which is usually January 1st itself.
        DateTime newYearsDay = new DateTime(currentDateTime.Year, 01, 01);
        DateTime bankHoliday = newYearsDay;
        while (IsWorkingDay(bankHoliday.DayOfWeek) == false)
        {
            bankHoliday = bankHoliday.AddDays(1);
        }

        return currentDateTime.Date == bankHoliday.Date;
    }

    public bool IsEasterBankHolidayMonday(DateTime currentDateTime)
    {
        // Easter bank holiday is always the Monday after Easter Sunday.
        DateTime easterSunday = GetEasterSunday(currentDateTime.Year);
        return easterSunday.AddDays(1).Date == currentDateTime.Date;
    }

    public DateTime GetEasterSunday(int year)
    {
        // From http://stackoverflow.com/a/2510411/21574
        int day = 0;
        int month = 0;

        int g = year % 19;
        int c = year / 100;
        int h = (c - (int)(c / 4) - (int)((8 * c + 13) / 25) + 19 * g + 15) % 30;
        int i = h - (int)(h / 28) * (1 - (int)(h / 28) * (int)(29 / (h + 1)) * (int)((21 - g) / 11));

        day = i - ((year + (int)(year / 4) + i + 2 - c + (int)(c / 4)) % 7) + 28;
        month = 3;

        if (day > 31)
        {
            month++;
            day -= 31;
        }

        return new DateTime(year, month, day);
    }

    public bool IsMayBankHolidayMonday(DateTime currentDateTime)
    {      
        // The first Monday of May is a bank holiday (May day)
        DateTime firstMayBankHoliday = new DateTime(currentDateTime.Year, 05, 01);
        while (firstMayBankHoliday.DayOfWeek != DayOfWeek.Monday)
        {
            firstMayBankHoliday = firstMayBankHoliday.AddDays(1);
        }

        if (currentDateTime.Date == firstMayBankHoliday.Date)
            return true;

        // The last Monday of May is a bank holiday (Spring bank holiday)
        DateTime lastMayBankHoliday = new DateTime(currentDateTime.Year, 05, 31);
        while (lastMayBankHoliday.DayOfWeek != DayOfWeek.Monday)
        {
            lastMayBankHoliday = lastMayBankHoliday.AddDays(-1);
        }

        if (currentDateTime.Date == lastMayBankHoliday.Date)
            return true;

        return false;
    }

    public bool IsAugustBankHolidayMonday(DateTime currentDateTime)
    {
        // The last Monday of August is a bank holiday
        DateTime augustBankHoliday = new DateTime(currentDateTime.Year, 08, 31);
        while (augustBankHoliday.DayOfWeek != DayOfWeek.Monday)
        {
            augustBankHoliday = augustBankHoliday.AddDays(-1);
        }

        if (currentDateTime.Date == augustBankHoliday.Date)
            return true;
        else
            return false;
    }

    public bool IsDecemberBankHoliday(DateTime currentDateTime)
    {
        // December's bank holiday is the first working day on, or after Boxing day.
        DateTime boxingDay = new DateTime(currentDateTime.Year, 12, 26);
        DateTime bankHoliday = boxingDay;

        if (boxingDay.DayOfWeek == DayOfWeek.Monday)
            bankHoliday = boxingDay.AddDays(1);

        while (IsWorkingDay(bankHoliday.DayOfWeek) == false)
        {
            bankHoliday = bankHoliday.AddDays(1);
        }

        if (currentDateTime.Date == bankHoliday.Date)
            return true;
        else
            return false;
    }

    public bool IsWorkingDay(DayOfWeek dayOfWeek)
    {
        if (dayOfWeek == DayOfWeek.Saturday || dayOfWeek == DayOfWeek.Sunday)
            return false;

        return true;
    }
}