How can I calculate what date Good Friday falls on

Question

Does anyone have a good algorithm to calculate what date Good Friday falls on given the year as an input? Preferably in C#.

Answer 1

Wikipedia knows: http://en.wikipedia.org/wiki/Good_Friday#Calculating_the_date

Good Friday is the Friday before Easter, which is calculated differently in Eastern Christianity and Western Christianity (see Computus for details). Easter falls on the first Sunday following the Paschal Full Moon, the full moon on or after 21 March, taken to be the date of the vernal equinox. The Western calculation uses the Gregorian calendar, while the Eastern calculation uses the Julian calendar, whose 21 March now corresponds to the Gregorian calendar's 3 April. The calculations for identifying the date of the full moon also differ. See Easter Dating Method (Astronomical Society of South Australia).

In Eastern Christianity, Easter can fall between March 22 and April 25 on Julian Calendar (thus between April 4 and May 8 in terms of the Gregorian calendar, during the period 1900 and 2099), so Good Friday can fall between March 20 and April 23, inclusive (or between April 2 and May 6 in terms of the Gregorian calendar). (See Easter.)

Answer 2

Necromancing.
Actually, it depends on whether it is Orthodox or Catholic Good Friday ;)

from https://mycodepad.wordpress.com/2013/04/28/c-calculating-orthodox-and-catholic-easter/

(Note: Easter = Easter Sunday)

/// <summary>
/// Get Orthodox easter for requested year
/// </summary>
/// <param name="year">Year of easter</param>
/// <returns>DateTime of Orthodox Easter</returns>
public static DateTime GetOrthodoxEaster( int year ) {
    int a = year % 19;
    int b = year % 7;
    int c = year % 4;

    int d = (19 * a + 16) % 30;
    int e = (2 * c + 4 * b + 6 * d) % 7;
    int f = (19 * a + 16) % 30;
    int key = f + e + 3;

    int month = (key > 30) ? 5 : 4;
    int day = (key > 30) ? key - 30 : key;

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


/// <summary>
/// Get Catholic easter for requested year
/// </summary>
/// <param name="year">Year of easter</param>
/// <returns>DateTime of Catholic Easter</returns>
public static DateTime GetCatholicEaster( int year ) {
    int month = 3;
    int G = year % 19 + 1;
    int C = year / 100 + 1;
    int X = (3 * C) / 4 - 12;
    int Y = (8 * C + 5) / 25 - 5;
    int Z = (5 * year) / 4 - X - 10;
    int E = (11 * G + 20 + Y - X) % 30;
    if (E == 24) { E++; }
    if ((E == 25) && (G > 11)) { E++; }
    int N = 44 - E;
    if (N < 21) { N = N + 30; }
    int P = (N + 7) - ((Z + N) % 7);
    if (P > 31) {
        P = P - 31;
        month = 4;
    }
    return new DateTime( year, month, P );
}

Then you could still create an abstract EasterBunny:

private static void EasterBunnyTest()
{
    AbstractEasterBunny WesternEuropeanBunny = new CatholicEasterBunny();
    AbstractEasterBunny EasternEuropeanBunny = new OrthodoxEasterBunny();
    AbstractEasterBunny LocalizedEasterBunny = AbstractEasterBunny.CreateInstance();


    System.DateTime dtRomeEaster = WesternEuropeanBunny.EasterSunday(2016);
    System.DateTime dtAthensEaster = EasternEuropeanBunny.EasterSunday(2016);
    System.DateTime dtLocalEaster = LocalizedEasterBunny.EasterSunday(2016);

    System.Console.WriteLine(dtRomeEaster);
    System.Console.WriteLine(dtAthensEaster);
    System.Console.WriteLine(dtLocalEaster);
}

With this abstract bunny here:

public abstract class AbstractEasterBunny
{

    /// <summary>
    /// Gets the Orthodox easter sunday for the requested year
    /// </summary>
    /// <param name="year">The year you want to know the Orthodox Easter Sunday of</param>
    /// <returns>DateTime of Orthodox Easter Sunday</returns>
    public abstract System.DateTime EasterSunday(int year);

    public abstract System.DateTime GoodFriday(int year);


    public static AbstractEasterBunny CreateInstance()
    {
        System.Globalization.CultureInfo ci = System.Globalization.CultureInfo.CurrentCulture;
        System.Globalization.RegionInfo ri = new System.Globalization.RegionInfo(ci.LCID);


        // https://msdn.microsoft.com/en-us/library/windows/desktop/dd374073(v=vs.85).aspx
        System.Collections.Generic.List<int> lsOrthodox = new System.Collections.Generic.List<int>{
             0x10D // Serbia and Montenegro
            ,0x10E // Montenegro
            ,0x10F // Serbia
            ,0x19 // Bosnia and Herzegovina

            // ,0x46 // Estonia
            // ,0x4B // Czech Republic
            // ,0x4D // Finland
            ,0x62 // Greece
            // ,0x6D // Hungary
            ,0x79 // Iraq
            // ,0x8C // Latvia
            // ,0x8D // Lithuania
            // ,0x8F // Slovakia
            // ,0x98 // Moldova
            // ,0xD4 // Slovenia
            ,0x4CA2 // Macedonia, Former Yugoslav Republic of
            ,0xEB // Turkey
        };

        // if(ci == WesternSlavonicOrthodox)
        if (lsOrthodox.Contains(ri.GeoId))
            return new OrthodoxEasterBunny();


        // TODO: Correct for Armenia/Georgia ? ? ? 
        // if(ri.GeoId == 0x7 || ri.GeoId == 0x58) // 0x7: Armenia, 0x58: Georgia
            // return new CatholicEasterBunny();


        // if(ci == EasternSlavonic)
        string strMonthName = ci.DateTimeFormat.GetMonthName(8);
        if (System.Text.RegularExpressions.Regex.IsMatch(strMonthName, @"\p{IsCyrillic}"))
        {
            // there is at least one cyrillic character in the string
            return new OrthodoxEasterBunny();
        }

        return new CatholicEasterBunny();
    }

}



public class OrthodoxEasterBunny : AbstractEasterBunny
{

    /// <summary>
    /// Gets the Orthodox easter sunday for the requested year
    /// </summary>
    /// <param name="year">The year you want to know the Orthodox Easter Sunday of</param>
    /// <returns>DateTime of Orthodox Easter Sunday</returns>
    public override System.DateTime EasterSunday(int year)
    {
        int a = year % 19;
        int b = year % 7;
        int c = year % 4;

        int d = (19 * a + 16) % 30;
        int e = (2 * c + 4 * b + 6 * d) % 7;
        int f = (19 * a + 16) % 30;
        int key = f + e + 3;

        int month = (key > 30) ? 5 : 4;
        int day = (key > 30) ? key - 30 : key;

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


    public override System.DateTime GoodFriday(int year)
    {
        return this.EasterSunday(year).AddDays(-2);
    }

}



public class CatholicEasterBunny : AbstractEasterBunny
{

    /// <summary>
    /// Gets the Catholic easter sunday for the requested year
    /// </summary>
    /// <param name="year">The year you want to know the Catholic Easter Sunday of</param>
    /// <returns>DateTime of Catholic Easter Sunday</returns>
    public override System.DateTime EasterSunday(int year)
    {
        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 System.DateTime(year, month, day);
    }


    public override System.DateTime GoodFriday(int year)
    {
        return this.EasterSunday(year).AddDays(-2);
    }

}

Answer 3

Is it possible to the use the Hebrew or Arabic lunar calendars for the conversion? eg:

 DateTime getEasterSunday(int year)
        {
            const int fourTeen = 14;
            DateTime Paschal = new DateTime(1900, 3, 20);
            var iCal = new HebrewCalendar();
            DateTime eFullMoon;
            var pDate = new DateTime(year, Paschal.Month, Paschal.Day);
            var LunarYear = iCal.GetYear(pDate);
            var LunarMonth = iCal.GetMonth(pDate);
            var LunarDay = iCal.GetDayOfMonth(pDate);

            if (LunarDay >= fourTeen) LunarMonth++;

            eFullMoon = iCal.ToDateTime(LunarYear, LunarMonth, fourTeen, 0, 0, 0, 0);

            return Enumerable.Range(0, 6).Select(x => eFullMoon.Date.AddDays(x)).Where(x => x.DayOfWeek == DayOfWeek.Sunday).First();
        }

Answer 4

Try this:

// test code:
Console.WriteLine(CalcGoodFri(2008));
Console.WriteLine(CalcGoodFri(2009));
Console.WriteLine(CalcGoodFri(2010));

private static DateTime CalcGoodFri(int yr)
{
 //int yr = 2010;  // The year for which to determine the date of Good Friday.
 int a = yr % 19;      
 int b = yr / 100;     
 int c = yr % 100;   
 int d = b / 4;
 int e = b % 4;      
 int i = c / 4;
 int k = c % 4;
 int g = (8 * b + 13) / 25;
 int h = ((19 * a) + b - d - g + 15) % 30;
 int l = ((2 * e) + (2 * i) - k + 32 - h) % 7;
 int m = (a + (11*h) + (19*l)) / 433;
 int days_to_good_friday = h + l - (7*m) - 2;  
 int mo = (days_to_good_friday + 90) / 25;
 int da = (days_to_good_friday + (33 * mo) + 19) % 32;
 return new DateTime ( yr, mo, da) ;    // Returns the date of Good Friday
}

Logic ported from here: http://www.kenhamady.com/form25.shtml

Answer 5

Here's a great article that should help you build your algorithm

http://www.codeproject.com/KB/datetime/christianholidays.aspx

Based on this example, you should be able to write:

DateTime goodFriday = EasterSunday(DateTime.Now.Year).AddDays(-2);

Full Example:

public static DateTime EasterSunday(int year)
{
    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);
}

Answer 6

Don't Repeat Yourself

Think

Realize that calculating Easter is what you are really dependent upon.

Research

Here is the offical Naval Observatory page for calculating Easter.

http://aa.usno.navy.mil/faq/docs/easter.php

Execute

Use the formula for calculating Easter then shift to the previous Friday (or subtract 2 days, details up to you).