
Sunday  Monday  Tuesday  Wednesday  Thursday  Friday  Saturday 
A  G  F  E  D  C  B 
While it is not hard to look at this December's calendar to figure what next year's Dominical Letter will be, the table below will be helpful to find the Letter for any year of interest since the Gregorian reform.
Having obtained the Dominical Letter (DL) it remains necessary to determine the "epact," which is simply the age of the moon in days on January 1st. The epact may be known by making reference to the 19 year cycle of the moon, discovered in 432 B.C. by the Greek astronomer Meton. The current point on the "Metonic" cycle is identified with the "golden number" (the ancient Greeks marked it on a pillar in gold paint). In the Missale Romanum, the golden number is called by the Latin term "aureus numererus." The golden number must be an integer between 1 and 19 inclusive, and is very easily computed:
Golden Number = (year + 1) MOD 19.
For those not familiar with modular division, this is the same as "add 1 to the year, divide by 19, and keep just the "remainder," which is the golden number. If it is not intuitively obvious, the reader is assured that (year +1) MOD 19 is exactly equal to (year) MOD 19 +1, as the formula is often given in secular references.
By way of example, the golden number for the year 2007 will be 2007+1 MOD 19. Or 2008/19 = 105 remainder 13, where the remainder, 13, is the golden number for 2007. If there is no remainder, the golden number is 19.
Before the Gregorian reform, the golden number would have sufficed to find the Easter date (in conjunction with the Sunday letter). But the essence of the reform was the attempt to bring the solar and lunar calendars into conformity with astronomical observation. Sometimes the lunar and solar corrections cancel each other, but every few centuries they do not, requiring that (for the few hundred years under consideration) a new table of epacts be related to the golden numbers.
Once the epact and Dominical Letter have been identified, it is easy to look up the date of Easter on the table of moveable feasts.
The following chart may be used to determine the DL for the years between 1600 and 2800 inclusive.
Last two digits of Year 
First two digits of Year 
16xx  
17xx  18xx  19xx  20xx  
21xx  22xx  23xx  24xx  
25xx  26xx  27xx  28xx  
Dominical Letters: 
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 
16xx  
17xx  18xx  19xx  20xx  
21xx  22xx  23xx  24xx  
25xx  26xx  27xx  28xx 
The "epact," the reader will remember, is the nineteen year lunar cycle. Two peculiarities are found in the epact tables. Sometimes we see an epact denoted by the asterisk (*). This is simply a zero  which was not in use until relatively modern times in the western world  indicating that the new moon falls on the first day of the year  it is zero days old. Apart from the "*," all of the epacts have been designated with roman numerals  printed in red, where red and black printing is available.
A second peculiarity will be found in the table below for A.D. 19002199. Since the lunar month is not an integral number of days long, the "Kalendarium" section of the Missal makes up for the discrepancy by "doubling up" epacts xxiv and xxv during the shorter months of the year. But sometimes this adjustment needs to be counter adjusted. The epact "25" in black Arabic numerals represents an epact greater than xxv and less than xxvi  "251/2," for lack of a way to represent fractions in Roman numerals. The "black 25" epact occurs whenever its corresponding golden number is from 12 to 19 (inclusive). Golden numbers 1 through 11 (inclusive) correspond to the epact "red xxv." Understanding the numerical value of "25" will help the reader to detect the printing errors that are sometimes found in the paschal tables of modern missals.
Before the Gregorian reform  from year 1 through October 1582 the epacts and golden numbers consistently followed the chart below (although the term "epact" was unnecessary and not used before the reform):
A.D. 1 through October A.D. 1582  Golden Number: 1 2 3 4 5 6 7 8 9 Epact xj xxij iij xiv xxv vi xvij xxviij ix Golden No: 10 11 12 13 14 15 16 17 18 19 Epact: xx j xij xxiij iv xv xxvj vij xviij xxix 
The Gregorian reform altered the Metonic cycle, leaving us with the epact table below for the days in 1582 after the October correction, until 1699 (inclusive):
A.D. 15821699 (inclusive)  Golden Number: 6 7 8 9 10 11 12 13 14 Epact: xxvj vij xviij xxix x xxj ij xiij xxiv Golden No: 15 16 17 18 19 1 2 3 4 5 Epact: v xvj xxvij viij xix j xij xxiij iv xv 
A.D. 1700 not being a leap year (a solar correction) brought a new table into play:
A.D. 17001899 (inclusive)  Golden Number: 10 11 12 13 14 15 16 17 18 19 Epact: ix xx j xij xxiij iv xv xxvj vij xviij Golden Number: 1 2 3 4 5 6 7 8 9 Epact: * xj xxij iij xiv xxv vj xvij xxviij  Epact Table: A.D. 19002199 (inclusive)  Golden Number: 1 2 3 4 5 6 7 8 9 Epact: xxix x xxj ij xiij xxiv v xvj xxvij Golden Number: 10 11 12 13 14 15 16 17 18 19 Epact: viij xix * xj xxij iij xiv 25 vj xvij  EASTER TABLE FOLLOWING THE GREGORIAN CALENDAR REFORM Adapted from Missale Romanum, "The Year and its Parts" Sunday Cycle of Septua Easter Sundays First Letter the Epact gesima Sunday after Advent (DL) Pentecost Sunday  D xxiii 18 Jan 22 Mar 28 29 Nov D xvixxii 25 Jan 29 Mar 27 29 Nov D ixxv 1 Feb 5 Apr 26 29 Nov D iiviii 8 Feb 12 Apr 25 29 Nov D *,i,xxivxxix,25 15 Feb 19 Apr 24 29 Nov  E xxiixxiii 19 Jan 23 Mar 28 30 Nov E xvxxi 26 Jan 30 Mar 27 30 Nov E viiixiv 2 Feb 6 Apr 26 30 Nov E ivii 9 Feb 13 Apr 25 30 Nov E xxivxxix,*,25 16 Feb 20 Apr 24 30 Nov  F xxixxiii 20 Jan 24 Mar 28 1 Dec F xivxx 27 Jan 31 Mar 27 1 Dec F viixiii 3 Feb 7 Apr 26 1 Dec F *,ivi 10 Feb 14 Apr 25 1 Dec F xxivxxix,25 17 Feb 21 Apr 24 1 Dec  G xxxxiii 21 Jan 25 Mar 28 2 Dec G xiiixix 28 Jan 1 Apr 27 2 Dec G vixii 4 Feb 8 Apr 26 2 Dec G xxix,*,iv 11 Feb 15 Apr 25 2 Dec G xivxviii,25 18 Feb 22 Apr 24 2 Dec  Sunday Cycle of Septua Easter Sundays First Letter the Epact gesima Sunday after Advent (DL) Pentecost Sunday  A xixxxiii 22 Jan 26 Mar 28 3 Dec A xiixviii 29 Jan 2 Apr 27 3 Dec A vxi 5 Feb 9 Apr 26 3 Dec A xxviii,xxix,*,iiv 12 Feb 16 Apr 25 3 Dec A xxivxxvii,25 19 Feb 23 Apr 24 3 Dec  B xviiixxiii 23 Jan 27 Mar 27 27 Nov B xixvii 30 Jan 3 Apr 26 27 Nov B ivx 6 Feb 10 Apr 25 27 Nov B xxviixxix,*,iiii 13 Feb 17 Apr 24 27 Nov B xxivxxvi,25 20 Feb 24 Apr 23 27 Nov  C xviixxiii 24 Jan 28 Mar 27 28 Nov C xxvi 31 Jan 4 Apr 26 28 Nov C iiiix 7 Feb 11 Apr 25 28 Nov C 25,xxvixxix,*,i,ii 14 Feb 18 Apr 24 28 Nov C xxiv,xxv 21 Feb 25 Apr 23 28 Nov  Sunday Cycle of Septua Easter Sundays First Letter the Epact gesima Sunday after Advent (DL) Pentecost Sunday
In leap years the saints' days that fall on February 2428 are observed on February 2529. In the cases where Septuagesima falls in January or early February, it and the following Sunday observances must be adjusted to fall on Sunday, even though they fall before the correction is made.
The "Kalendarium" section of the Missale Romanum is a listing of each of the days of the months of the year as observed in the Proper of Saints  beginning with the feast of the Circumcision of Our Lord on January 1st, and ending with the feast of Pope Saint Sylvester on December 31st. The "Kalendarium" is somewhat like the secular calendar in that it makes no mention of Easter and the moveable feasts. It must be integrated with the moveable feasts in order to produce the ecclesiastical calendar for any given year. Nonetheless, the way it is printed in the Missal  with a consecutive epact and Sunday (Dominical) letter printed in front of each day's date  it can be used to determine the phase of the moon, and the days of the year that fall on Sundays.
In the "Kalendarium" section, the new moon falls on the days corresponding to the year's epact. If, for example, the epact for the year is "xxiv," all of the days in the "Kalendarium" having this number prefixed will be the days of the new moon. And, also by way of example, if the Sunday Letter (DL) is "e," the days with an "e" prefixed will all be Sundays. Do not forget that the DL changes after the leap year correction is made in years with two DLs.
NOTES:
Much of the information contained herein has been adapted from the section, "The Year and Its Parts" in the Missale Romanum. The Catholic Encyclopedia, s.v. "epact," has tables through the year 5000 A.D.
An excellent reference for the history and science of calendar creation is Faith Wells, Bede: The Reckoning of Time (Liverpool: Liverpool University Press, 1999), available through the University of Pennsylvania Press in these United States. Dr. Wells' introduction and notes on translation are worth the price of the book, and the Venerable Bede gives an impressive example of scientific analysis conducted in the middle ages.
(1) The 14th day gave its name to the debate: the "Quartodecemian" controversy.
(2) See The Our Lady of the Rosary Parish Bulletin, March 1999, for an
explanation of the Gregorian calender reform.