I was needing to check for Holidays in an Appointment calendar and found some decent code that needed a bit of refactoring.

http://geekswithblogs.net/wpeck/archive/2011/12/27/us-holiday-list-in-c.aspx

static HashSet<DateTime> GetHolidays(int year) {

    var holidays = new HashSet<DateTime>{
        NewYears(year),
        MemorialDay(year),
        IndependenceDay(year),
        LaborDay(year),
        Thanksgiving(year),
        Christmas(year)
    };

    var next = NextNewYears(year);

    if(next != null)
        holidays.Add(next.Value);

    return holidays;
}

static DateTime NewYears(int year){
    return AdjustForWeekendHoliday(new DateTime(year, 1, 1));
}

static DateTime IndependenceDay(int year){
    return AdjustForWeekendHoliday(new DateTime(year, 7, 4));
}

/// 
<summary>
/// Memorial Day is the last Monday in May 
/// </summary>

/// <param name="year"></param>
/// <returns></returns>

static DateTime MemorialDay(int year){

    var day = new DateTime(year, 5, 31);

    while (day.DayOfWeek != DayOfWeek.Monday) {
        day = day.AddDays(-1);
    }

    return day;
}

/// 
<summary>
/// Labor Day is the first Monday in September 
/// </summary>

/// <param name="year"></param>
/// <returns></returns>

static DateTime LaborDay(int year)
{
    var day = new DateTime(year, 9, 1);

    while (day.DayOfWeek != DayOfWeek.Monday) {
        day = day.AddDays(1);
    }

    return day;
}

/// 
<summary>
/// Thanksgiving Day is the fourth Thursday in November 
/// </summary>

/// <param name="year"></param>
/// <returns></returns>
        
static DateTime Thanksgiving(int year)
{
    var thanksgiving = (from date in Enumerable.Range(1, 30)
        where new DateTime(year, 11, date).DayOfWeek == DayOfWeek.Thursday
        select date).ElementAt(3);

    var day = new DateTime(year, 11, thanksgiving);

    return day;
}

static DateTime Christmas(int year){
    return AdjustForWeekendHoliday(new DateTime(year, 12, 25));
}

/// 
<summary>
/// If next year's new years is adjusted for this year, then count it
/// </summary>

/// <param name="year"></param>
/// <returns></returns>

static DateTime? NextNewYears(int year){
    var day = AdjustForWeekendHoliday(new DateTime(year + 1, 1, 1));

    if (day.Year == year)
        return day;

    return null;
}

/// 
<summary>
/// If the holiday falls on a Saturday, then make it Friday.
/// If the holiday falls on a Sunday, then make it Monday.
/// </summary>

/// <param name="holiday"></param>
/// <returns></returns>

public static DateTime AdjustForWeekendHoliday(DateTime holiday) {
    if (holiday.DayOfWeek == DayOfWeek.Saturday) {
        return holiday.AddDays(-1);
    }

    if (holiday.DayOfWeek == DayOfWeek.Sunday) {
        return holiday.AddDays(1);
    }

    return holiday;
}