Calculation of Easter
See also - Easter calculator
The festival of Easter is celebrated by both the Christian and Jewish religions. In Judaism, Easter is more commonly called Pesach and always falls on 15 Nisan of the Jewish calendar - around March or April in the Gregorian (Western) calendar. What date Christians celebrate Easter depends on where you live - the Eastern Orthodox Church sometimes celebrates Easter on a different day to that of the Westen Church. In what follows, when I refer to "Easter", I refer to the Easter of the Western Church.
When can Easter fall?
Easter can fall on any date (provided it is a Sunday) between 22nd March and 25th April inclusive.
What is the most likely date for Easter?
The most likely date for Easter is 19th April, which has a 3.87% chance of this happening in a given year.
What is the "average" date on which Easter falls?
The average (median) date on which Easter falls is 7th April. There is a 3.33% chance of this happening in a given year.
Why these dates?
Easter was originally defined as "the first Sunday after the first full moon on or after the March equinox", with the words March equinox meaning March 21st. Put more simply, to find the date of Easter, you look for the date of the first full moon that falls on or after March 21st. If the date of the full moon is a Sunday, then Easter Sunday will be on the following Sunday. If the date of the full moon is not a Sunday, Easter will be on the Sunday after the full moon.
Does this rule always hold true?
No. In the calculation of Easter, a standardised (or "average") full moon is used, which evens out small differences and makes it possible to calculate the dates for the years ahead.
So how is Easter calculated?
The date of Easter can be calculated using a simple algorithm (set of calculations) which involve looking at the quotients and remainders of a series of divisions. This algorithm is the Meeus/Jones/Butcher algorithm that can be found on Wikipedia.
Does this algorithm always hold true?
Yes. No exceptions, provided you are dealing with the Gregorian calendar (the calendar currently in use in the Western world).
How long does it take for the dates of Easter to start repeating themselves?
Under the present system (the Gregorian calendar), the full Easter cycle takes 5 700 000 years. Therefore as Easter falls on 16th April in 2006, it would also fall on 16th April 5 702 006 if the Gregorian calendar were still in use (though this is unlikely).
What other dates does Easter have an impact on?
All the other dates related to the festival of Easter and Lent - so Shrove Tuesday, Ash Wednesday, Maundy Thursday, Good Friday, Holy Saturday, and Easter Monday. The date of Easter also directly affects the date of Mothers' Day in the United Kingdom and Ireland (though in the United States, Mothers' Day always falls on the second Sunday of May).
So what are the extreme dates for these days?
When can Easter fall?
Easter can fall on any date (provided it is a Sunday) between 22nd March and 25th April inclusive.
What is the most likely date for Easter?
The most likely date for Easter is 19th April, which has a 3.87% chance of this happening in a given year.
What is the "average" date on which Easter falls?
The average (median) date on which Easter falls is 7th April. There is a 3.33% chance of this happening in a given year.
Why these dates?
Easter was originally defined as "the first Sunday after the first full moon on or after the March equinox", with the words March equinox meaning March 21st. Put more simply, to find the date of Easter, you look for the date of the first full moon that falls on or after March 21st. If the date of the full moon is a Sunday, then Easter Sunday will be on the following Sunday. If the date of the full moon is not a Sunday, Easter will be on the Sunday after the full moon.
Does this rule always hold true?
No. In the calculation of Easter, a standardised (or "average") full moon is used, which evens out small differences and makes it possible to calculate the dates for the years ahead.
So how is Easter calculated?
The date of Easter can be calculated using a simple algorithm (set of calculations) which involve looking at the quotients and remainders of a series of divisions. This algorithm is the Meeus/Jones/Butcher algorithm that can be found on Wikipedia.
Does this algorithm always hold true?
Yes. No exceptions, provided you are dealing with the Gregorian calendar (the calendar currently in use in the Western world).
How long does it take for the dates of Easter to start repeating themselves?
Under the present system (the Gregorian calendar), the full Easter cycle takes 5 700 000 years. Therefore as Easter falls on 16th April in 2006, it would also fall on 16th April 5 702 006 if the Gregorian calendar were still in use (though this is unlikely).
What other dates does Easter have an impact on?
All the other dates related to the festival of Easter and Lent - so Shrove Tuesday, Ash Wednesday, Maundy Thursday, Good Friday, Holy Saturday, and Easter Monday. The date of Easter also directly affects the date of Mothers' Day in the United Kingdom and Ireland (though in the United States, Mothers' Day always falls on the second Sunday of May).
So what are the extreme dates for these days?
Date
Shrove Tuesday
Ash Wednesday
Mothers' Day (UK and Ireland)
Palm Sunday
Maundy Thursday
Good Friday
Holy Saturday
Easter Sunday
Easter Monday
Shrove Tuesday
Ash Wednesday
Mothers' Day (UK and Ireland)
Palm Sunday
Maundy Thursday
Good Friday
Holy Saturday
Easter Sunday
Easter Monday
Earliest possible date
3rd February
4th February
1st March
15th March
19th March
20th March
21st March
22nd March
23rd March
3rd February
4th February
1st March
15th March
19th March
20th March
21st March
22nd March
23rd March
Latest possible date
9th March
10th March
4th April
18th April
22nd April
23rd April
24th April
25th April
26th April
9th March
10th March
4th April
18th April
22nd April
23rd April
24th April
25th April
26th April
Interestingly, Shrove Tuesday/Ash Wednesday can fall on 29th February in a leap year if Easter Sunday falls on the 16th April/15th April respectively.
Sources:
Sources:
- Astronomical Algorithms - Jean Meeus
- Days, Months and Years - Magdalen Bear
