How can I calculate what date Good Friday falls on given a year?

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

There's an online calculator here:

http://mb-soft.com/believe/txx/easter01.htm

It's written in pure javascript, and the script source is right there in the .html file. I'd mine it for algorithms and translate as appropriate.

Answer 2

http://www.assa.org.au/edm.html#Calculator

Answer 3

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(month, day, year);
}

Answer 4

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 5

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).

Answer 6

It's not C# but you might want to have a look here.

Answer 7

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 8

A quick note to non-US coders trying out Hunter's code: for the last line you should use ISO constructor new DateTime(year, month, day); :)