Dominical Letter, a device adopted from the Romans by the old chronologers to aid them in finding the day of the week corresponding to any given date, and indirectly to facilitate the adjustment of the “Proprium de Tempore” to the “Proprium Sanetorum” when constructing the ecclesiastical calendar for any year. The Church, on account of her complicated system of movable and immovable feasts (see Christian Calendar), has from an early period taken upon herself as a special charge to regulate the measurement of time. To secure uniformity in the observance of feasts and fasts, she began, even in the patristic age, to supply a computus, or system of reckoning, by which the relation of the solar and lunar years might be accommodated and the celebration of Easter determined. Naturally she adopted the astronomical methods then available, and these methods and the terminology belonging to them, having become traditional, are perpetuated in a measure to this day, even after the reform of the calendar, in the prolegomena to the Breviary and Missal.
The Romans were accustomed to divide the year into nundinoe, periods of eight days; and in their marble fasti, or calendars, of which numerous specimens remain, they used the first eight letters of the alphabet to mark the days of which each period was composed. When the Oriental seven-day period, or week, was introduced, in the time of Augustus, the first seven letters of the alphabet were employed in the same way to indicate the days of this new division of time. In fact, fragmentary calendars on marble still survive in which both a cycle of eight letters—A to H—indicating nundinoe, and a cycle of seven letters—A to G—indicating weeks, are used side by side (see “Corpus Inscriptionum Latinarum”, 2nd ed., I, 220.—The same peculiarity occurs in the Philocalian Calendar of A.D. 356, ibid., p. 256). This device was imitated by the Christians, and in their calendars the days of the year from January 1 to December 31 were marked with a continuous recurring cycle of seven letters: A, B, C, D, E, F, G. A was always set against January 1, B against January 2, C against January 3, and. so on. Thus F fell to January 6, G to January 7; A again recurred on January 8, and also, consequently, on January 15, January 22, and January 29. Continuing in this way, January 30 was marked with a B, January 31 with a C, and February 1 with a D. Supposing this to be carried on through all the days of an ordinary year (i.e. not a leap year), it will be found that a D corresponds to March 1, G to April 1, B to May 1, E to June 1, G to July 1, C to August 1, F to September 1, A to October 1, D to November 1, and F to December 1—a result which Durandus recalled by the following distich:
Alta Domat Dominus, Gratis Beat Equa Gerentes
Contemnit Fictos, Augebit Dona Fideli.
Now, as a moment’s reflection shows, if January 1 is a Sunday, all the days marked by A will also be Sundays; if January 1 is a Saturday, Sunday will fall on January 2, which is a B, and all the other days marked B will be Sundays; if January 1 is a Monday, then Sunday will not come until January 7, a G, and all the days marked by a G will be Sundays. This being explained, the Dominical Letter of any year is defined to be that letter of the cycle, A, B, C, D, E, F, G, which corresponds to the day upon which the first Sunday (and every subsequent Sunday) falls.
It is plain, however, that when leap year occurs, a complication is introduced. February has then twenty-nine days. According to the Anglican and civil calendars this extra day is added to the end of the month; according to the Catholic ecclesiastical calendar February 24 is counted twice. But in either case March 1 is then one day later in the week than February 1, or, in other words, for the rest of the year the Sundays come a day earlier than they would in a common year. This is expressed by saying that a leap year has two Dominical Letters, the second being the letter which precedes that with which the year started. For example, January 1, 1907, was a Tuesday; the first Sunday fell on January 6, or an F. F was, therefore, the Dominical Letter for 1907. The first of January, 1908, was a Wednesday, the first Sunday fell on January 5, and E was the Dominical Letter, but as 1908 was leap year, its Sundays after February came a day sooner than in the normal year and were D’s. The year 1908, therefore, had a double Dominical Letter, E-D. In 1909, January 1 is a Friday and the Dominical Letter is C. In 1910 and 1911, January 1 falls respectively on Saturday and Sunday and the Dominical Letters are B and A.
This, of course, is all very simple, but the advantage of the device lies, like that of an algebraical expression, in its being a mere symbol adaptable to any year. By constructing a table of letters and days of the year, A always being set against January 1, we can at once see the relation between the days of the week and the day of any month, if only we know the Dominical Letter. This may always be found by the following rule of De Morgan’s, which gives the Dominical Letter for any year, or the second Dominical Letter if it be leap year: I. Add 1 to the given year.
II. Take the quotient found by dividing the given year by 4 (neglecting the remainder).
III. Take 16 from the centurial figures of the given year if that can be done.
IV. Take the quotient of III divided by 4 (neglecting the remainder). V. From the sum of I, II, and IV, subtract III.
VI. Find the remainder of V divided by 7: this is the number of the Dominical Letter, supposing A, B, C, D, E, F, G to be equivalent respectively to 6, 5, 4, 3, 2, 1, 0.
For example, to find the Dominical Letter of the year 1913:—
(I, II, IV) 1914 + 478 + 0 = 2392
(III) 19-16 = 3
(V) 2392-3 = 2389
(VI) 2389 ÷ 7 = 341, remainder 2.
Therefore the Dominical Letter is E.
But the Dominical Letter had another very practical use in the days before the “Ordo divini officii recitandi” was printed annually and when, consequently, a priest had often to determine the “Ordo” for himself (see Catholic Directories). As will be shown in the articles Epact and Easter Controversy. Easter Sunday may be as early as March 22 or as late as April 25, and there are consequently thirty-five possible days on which it may fall. It is also evident that each Dominical Letter allows five possible dates for Easter Sunday. Thus, in a year whose Dominical Letter is A (i.e. when January 1 is a Sunday), Easter must be either on March 26, April 2, April 9, April 16, or April 23, for these are all the Sundays within the defined limits. But according as Easter falls on one or another of these Sundays we shall get a different calendar, and hence there are five, and only five, possible calendars for years whose Dominical Letter is A. Similarly, there are five possible calendars for years whose Dominical Letter is B, five for C, and so on, thirty-five possible combinations in all. Now, advantage was taken of this principle in the arrangement of the old Pye or directorium which preceded our present “Ordo”. The thirty-five possible calendars were all included therein and numbered, respectively, primum A, secundum A, tertium A, etc.; primum B, secundum B, etc. Hence for anyone wishing to use the Pye the first thing to determine was the Dominical Letter of the year, and then by means of the Golden Number or the Epact, and by the aid of a simple table, to find which of the five possible calendars assigned to that Dominical Letter belonged to the year in question. Such a table as that just referred to, but adapted to the reformed calendar and in more convenient shape, will be found at the beginning of every Breviary and Missal under the heading, “Tabula Paschalis nova reformata”.
The Dominical Letter does not seem to have been familiar to Bede in his “De Temporum Ratione”, but in its place he adopts a similar device of seven numbers which he calls concurrentes (De Temp. Rat., cap. liii). This was of Greek origin. The Concurrents are numbers denoting the day of the week on which March 24 falls in the successive years of the solar cycle, 1 standing for Sunday, 2 (feria secunda) for Monday,3 for Tuesday, and so on. It is sufficient here to state that the relation between the Concurrents and the Dominical Letter is the following:
Concurrents………………..1 2 3 4 5 6 7
Dominical Letter…………..F E D C B A G