CALCULATING EASTER

Largely adopted from http://www.shagtown.com a deeply researched site site devoted to calendars and holidays

Lent

Lent is the penitential season preceding Easter, observed with forty days of fast in memory of Christ's forty days of fast in the desert. Lent consists of forty weekdays and six Sundays. The name is derived from the Middle English Lente, and refers to the lengthening of the daylight hours. [eating fish is a holdover from pagan things]

Quadragesima, first Sunday of Lent. May also mean the forty days of Lent.
Orthodoxy Sunday, first Sunday of Lent. Commemorates the restoration of the use of icons in the church (842 AD), and the triumph over all heresies.
Laetare Sunday, fourth Sunday of Lent. Takes its name from the introit of that day which begins with "Laetare Jerusalem" (Rejoice ye, Jerusalem). Also known as Mid-Lent Sunday. In England it is called Mothering Sunday, and has its own customs.
Passion Sunday, fifth Sunday in Lent . Two weeks before Easter. Also known as Judica.
Passiontide. The two-week period from Passion Sunday to Holy Saturday.
Passion Week. May refer to the week preceding Easter, also known as Holy Week, or the week before Holy Week beginning with Passion Sunday.

Holy Week

Holy Week is the week preceding Easter Sunday.

Palm Sunday, Sunday before Easter . Commemorates Christ's entry into Jerusalem.
Maundy Thursday, Thursday before Easter . Commemorates Christ's Last Supper and His washing of the disciples' feet on that day.
Good Friday, Friday before Easter (Apr 9, 2004). The anniversary of the crucifixion of Christ. Also known as Holy Friday.
Holy Saturday, day before Easter .
Holy Thursday. Variously confused with Ascension Day or Maundy Thursday.

Easter

Easter is the celebration of Christ's resurrection. The resurrection took place on Sunday, which was from then on the "Lord's Day".

After Easter

Rogation Sunday, Sunday before Ascension Day. The Latin name is Vocem Juncunditatis. Rogation Days are the three days preceding Ascention Day.
Ascension Day, ten days before Pentecost Commemorates the Ascension of Christ. A public holiday in may Roman Catholic nations. Observed in some Carribean nations on May 12.
Pentecost, seventh Sunday after Easter A festival commemorating the descent of the Holy Ghost upon the apostles. Also known as Whitsunday, meaning "white Sunday", probably due to the white baptismal robes worn on that day. The origins are linked to the ancient Roman festival of Floralia.
Whitsuntide. The week beginning with Whitsunday, especially the first three days of this week. Also known as Whit Week.
Whitmonday, day after Whitsunday. A holiday in Great Britain.
Trinity Sunday, Sunday after Pentecost . A festival in honor of the Trinity.
Corpus Christi, Thursday after Trinity Sunday A festival in honor of the Eucharist, or Lord's Supper. The name means "body of Christ". A public holiday in many Roman Catholic nations, especially in South America. In the United States, Corpus Christi is observed on the Sunday after Trinity Sunday.

In the Sixteenth Century, Aloysius Lilius, an author of the Gregorian Calendar, developed a method of calculating Easter which does not depend on the actual moon phases.

the epact is used for determining the moon phases. Because Easter is always on a Sunday, the dominical letter

is also part of the equation. For leap years use the second letter. Use the epact and dominical letter for any year to look up the date of Easter in the table.

To determine the date of Orthodox Easter, use this trick: reset epact to (epact+9)%30, and use the next dominical letter (so that A becomes B, and G becomes A). This will get the date on the Julian calendar. For the corresponding date on the Gregorian calendar, add 13 days.

This method uses the epact to estimate the age of the new moon, and the dominical letter to determine the day of the week. It also fixes the Spring equinox to be on March 21st regardless of the date of the actual equinox. You can calculate the epact mathematically, or look it up in a table. Likewise with the dominical letter. Once you have the epact and dominical letter, you can look up Easter Sunday in a table. Using the epact is the official method for determining Easter even when the actual full moon would give a different date.

The age of the new moon at the beginning of the year. This number is used in this encyclopedia to determine the dates of observances which are related to phases of the moon (such as Easter).

The epact may be determined for years 1582 to 4200 as follows. Equation for epact.

Where N is the "golden number", and c is the century (e.g. 19 for 1999). First calculate N by adding 1 to the year, dividing by 19, and taking the remainder. Also use only the remainder when dividing by 30. When dividing the century terms by 3 or 4, discard the fractions.

To look up the epact for any year using the table, find the century in the left column, and the last two digits of the year in the top row. Find the intersecting cell, and that is the epact for the year. For example, the epact for 1965 is in the row for 1900 and the column for 65. The epact for 1965 is 27.

Century	Last Two Digits of Year
Century	00 19 38 57 76 95	01 20 39 58 77 96	02 21 40 59 78 97	03 22 41 60 79 98	04 23 42 61 80 99	05 24 43 62 81	06 25 44 63 82	07 26 45 64 83	08 27 46 65 84	09 28 47 66 85	10 29 48 67 86	11 30 49 68 87	12 31 50 69 88	13 32 51 70 89	14 33 52 71 90	15 34 53 72 91	16 35 54 73 92	17 36 55 74 93	18 37 56 75 94

1600	15	26	7	18	29	10	21	2	13	24	5	16	27	8	19	0	11	22	3
1700	9	20	1	12	23	4	15	26	7	18	29	10	21	2	13	24	5	16	27
1800	4	15	26	7	18	0	11	22	3	14	25	6	17	28	9	20	1	12	23
1900	29	10	21	2	13	24	5	16	27	8	19	0	11	22	3	14	25	6	17
2000	24	5	16	27	8	19	0	11	22	3	14	25	6	17	28	9	20	1	12
2100	19	0	11	22	3	14	25	6	17	28	9	20	1	12	23	4	15	26	7
2200	13	24	5	16	27	8	19	0	11	22	3	14	25	6	17	28	9	20	1
2300	8	19	0	11	22	3	14	25	6	17	28	9	20	1	12	23	4	15	26
2400	4	15	26	7	18	0	11	22	3	14	25	6	17	28	9	20	1	12	23
2500	28	9	20	1	12	23	4	15	26	7	18	29	10	21	2	13	24	5	16
2600	22	3	14	25	6	17	28	9	20	1	12	23	4	15	26	7	18	29	10
2700	18	29	10	21	2	13	24	5	16	27	8	19	0	11	22	3	14	25	6
2800	13	24	5	16	27	8	19	0	11	22	3	14	25	6	17	28	9	20	1
2900	7	18	29	10	21	2	13	24	5	16	27	8	19	0	11	22	3	14	25
3000	2	13	24	5	16	27	8	19	0	11	22	3	14	25	6	17	28	9	20

dominical letter.

January
A	B	C	D	E	F	G
1	2	3	4	5	6	7
8	9	10	11	12	13	14
15	16	17	18	19	20	21
22	23	24	25	26	27	28
29	30	31

One of the letters from A to G used to indicate the Sundays through any particular year. If the columns of a calendar are marked with A through G, and January 1 always begins in the first column, then the dominical letter marks the column in which all Sundays fall. If the year begins on a Sunday, then the dominical letter is A. If the year begins on a Saturday, then the dominical letter is B, and so on. Leap years have a double dominical letter. The first letter is for days up to leap day (February 29), and the second letter is for the remaining days. The following tool gives the dominical letter for any year after 1582.

The dominical letter can be determined mathematically for any year after 1582 using the following equation.

Where x is the year and c is the number of the century (e.g. 19 for 1985). Disregard fractions in the division, then take the remainder when dividing by 7 (the number of days in the week). The result is the number of the dominical letter, where A = 1, B = 2, etc.

Last two digits of year	Century
				1600
	1700	1800	1900	2000
	2100	2200	2300	2400
	2500	2600	2700	2800
	2900	3000	3100	3200
00	C	E	G	BA
01 29 57 85	B	D	F	G
02 30 58 86	A	C	E	F
03 31 59 87	G	B	D	E
04 32 60 88	FE	AG	CB	DC
05 33 61 89	D	F	A	B
06 34 62 90	C	E	G	A
07 35 63 91	B	D	F	G
08 36 64 92	AG	CB	ED	FE
09 37 65 93	F	A	C	D
10 38 66 94	E	G	B	C
11 39 67 95	D	F	A	B
12 40 68 96	CB	ED	GF	AG
13 41 69 97	A	C	E	F
14 42 70 98	G	B	D	E
15 43 71 99	F	A	C	D
16 44 72	ED	GF	BA	CB
17 45 73	C	E	G	A
18 46 74	B	D	F	G
19 47 75	A	C	E	F
20 48 76	GF	BA	DC	ED
21 49 77	E	G	B	C
22 50 78	D	F	A	B
23 51 79	C	E	G	A
24 52 80	BA	DC	FE	GF
25 53 81	G	B	D	E
26 54 82	F	A	C	D
27 55 83	E	G	B	C
28 56 84	DC	FE	AG	BA

Epact	Dominical Letter
Epact	A	B	C	D	E	F	G
0	Apr 16	Apr 17	Apr 18	Apr 19	Apr 20	Apr 14	Apr 15
1	Apr 16	Apr 17	Apr 18	Apr 19	Apr 13	Apr 14	Apr 15
2	Apr 16	Apr 17	Apr 18	Apr 12	Apr 13	Apr 14	Apr 15
3	Apr 16	Apr 17	Apr 11	Apr 12	Apr 13	Apr 14	Apr 15
4	Apr 16	Apr 10	Apr 11	Apr 12	Apr 13	Apr 14	Apr 15
5	Apr 9	Apr 10	Apr 11	Apr 12	Apr 13	Apr 14	Apr 15
6	Apr 9	Apr 10	Apr 11	Apr 12	Apr 13	Apr 14	Apr 8
7	Apr 9	Apr 10	Apr 11	Apr 12	Apr 13	Apr 7	Apr 8
8	Apr 9	Apr 10	Apr 11	Apr 12	Apr 6	Apr 7	Apr 8
9	Apr 9	Apr 10	Apr 11	Apr 5	Apr 6	Apr 7	Apr 8
10	Apr 9	Apr 10	Apr 4	Apr 5	Apr 6	Apr 7	Apr 8
11	Apr 9	Apr 3	Apr 4	Apr 5	Apr 6	Apr 7	Apr 8
12	Apr 2	Apr 3	Apr 4	Apr 5	Apr 6	Apr 7	Apr 8
13	Apr 2	Apr 3	Apr 4	Apr 5	Apr 6	Apr 7	Apr 1
14	Apr 2	Apr 3	Apr 4	Apr 5	Apr 6	Mar 31	Apr 1
15	Apr 2	Apr 3	Apr 4	Apr 5	Mar 30	Mar 31	Apr 1
16	Apr 2	Apr 3	Apr 4	Mar 29	Mar 30	Mar 31	Apr 1
17	Apr 2	Apr 3	Mar 28	Mar 29	Mar 30	Mar 31	Apr 1
18	Apr 2	Mar 27	Mar 28	Mar 29	Mar 30	Mar 31	Apr 1
19	Mar 26	Mar 27	Mar 28	Mar 29	Mar 30	Mar 31	Apr 1
20	Mar 26	Mar 27	Mar 28	Mar 29	Mar 30	Mar 31	Apr 1
21	Mar 26	Mar 27	Mar 28	Mar 29	Mar 30	Mar 31	Apr 1
22	Mar 26	Mar 27	Mar 28	Mar 29	Mar 30	Mar 31	Apr 1
23	Mar 26	Mar 27	Mar 28	Mar 29	Mar 30	Mar 31	Apr 1
24	Apr 23	Apr 24	Apr 25	Apr 19	Apr 20	Apr 21	Apr 22
25	Apr 23	Apr 24	Apr 25	Apr 19	Apr 20	Apr 21	Apr 22
26	Apr 23	Apr 24	Apr 25	Apr 19	Apr 20	Apr 21	Apr 22
27	Apr 23	Apr 17	Apr 25	Apr 19	Apr 20	Apr 21	Apr 22
28	Apr 16	Apr 17	Apr 25	Apr 19	Apr 20	Apr 21	Apr 22
29	Apr 16	Apr 17	Apr 25	Apr 19	Apr 20	Apr 21	Apr 15

Easter may occur on any Sunday between March 22 and April 25, inclusive. We say that it is on the first Sunday after the first full moon following the spring equinox, but calculating the moon phases and equinox can be difficult, especially for Medeival priests. Therefore, for purposes of calculating Easter, the spring equinox is always considered to be on March 21, and the epact is used for determining the moon phases. Because Easter is always on a Sunday, the dominical letter is also part of the equation. For leap years use the second letter. Use the epact and dominical letter for any year to look up the date of Easter in the table.

The annual celebration of a person's birth probably originated in Egypt, where the birthdays of rulers and gods were celebrated with feasts. The early Christians did not celebrate birthdays at all.

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