Business/Holiday date handling

Nager.Date supports over 110 countries (US, DE, FR, RU, UK, ...) the library is available for .net45 and .netstandard 2.0. The list of supported countries can be found here.

Nuget

PM> install-package Nager.Date

Example:

Get all publicHolidays of a year

var publicHolidays = DateSystem.GetPublicHoliday(2018, CountryCode.DE);

Check if a date a public holiday

var date = new DateTime(2018, 01, 01);
if (DateSystem.IsPublicHoliday(date, CountryCode.DE))
{
    //Yes - New Year's Day
}

A little late to the party, but figure it will help other people googling it. Found a good answer at http://web.archive.org/web/20210128001521/http://geekswithblogs.net/wpeck/archive/2011/12/27/us-holiday-list-in-c.aspx. It even takes into account holidays that fall on a weekend.

private static HashSet<DateTime> GetHolidays(int year) 
{ 
    HashSet<DateTime> holidays = new HashSet<DateTime>();

    // New Years
    DateTime newYearsDate = AdjustForWeekendHoliday(new DateTime(year, 1, 1)); 
    holidays.Add(newYearsDate); 

    // Memorial Day -- last monday in May 
    DateTime memorialDay = new DateTime(year, 5, 31); 
    DayOfWeek dayOfWeek = memorialDay.DayOfWeek; 
    while (dayOfWeek != DayOfWeek.Monday) 
    { 
        memorialDay = memorialDay.AddDays(-1); 
        dayOfWeek = memorialDay.DayOfWeek; 
    } 
    holidays.Add(memorialDay); 
    
    // Independence Day
    DateTime independenceDay = AdjustForWeekendHoliday(new DateTime(year, 7, 4)); 
    holidays.Add(independenceDay); 
 
    // Labor Day -- 1st Monday in September 
    DateTime laborDay = new DateTime(year, 9, 1); 
    dayOfWeek = laborDay.DayOfWeek; 
    while(dayOfWeek != DayOfWeek.Monday) 
    { 
        laborDay = laborDay.AddDays(1); 
        dayOfWeek = laborDay.DayOfWeek; 
    } 
    holidays.Add(laborDay);

    // Thanksgiving Day -- 4th Thursday in November 
    var thanksgiving = (from day in Enumerable.Range(1, 30) 
                   where new DateTime(year, 11, day).DayOfWeek == DayOfWeek.Thursday 
                   select day).ElementAt(3); 
    DateTime thanksgivingDay = new DateTime(year, 11, thanksgiving); 
    holidays.Add(thanksgivingDay);

    // Christmas Day 
    DateTime christmasDay = AdjustForWeekendHoliday(new DateTime(year, 12, 25)); 
    holidays.Add(christmasDay); 

    // Next year's new years check
    DateTime nextYearNewYearsDate = AdjustForWeekendHoliday(new DateTime(year + 1, 1, 1));
    if (nextYearNewYearsDate.Year == year)
    holidays.Add(nextYearNewYearsDate);

    return holidays; 
}

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