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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 Sanctorum" 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 methodology belonging to them, having become traditional, are perpetuated in a measure to this day, even the reform of the calendar, in the prolegomena to the Breviary and Missal.
The Romans were accustomed to divide the year into nundinae, 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 nundinae, 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 1 January to 31 December were marked with a continuous recurring cycle of seven letters: A, B, C, D, E F, G. A was always set against 1 January, B against 2 January, C against 3 January, and so on. Thus F fell to 6 January, G to 7 January; A again recurred on 8 January, and also, consequently, on 15 January, 22 January, and 29 January. Continuing in this way, 30 January was marked with a B, 31 January with a C, and 1 February 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 1 March, G to 1 April, B to 1 May, E to 1 June, G to 1 July, C to 1 August, F to 1 September, A to 1 October, D to 1 November, and P to 1 December -- 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=92s reflection shows, if 1 January is a Sunday, all the days marked by A will also be Sundays; If 1 January is a Saturday, Sunday will fall on 2 January which is a B, and all the other days marked B will be Sundays; if 1 January is a Monday, then Sunday will not come until 7 January, a G, and all the days marked 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 a leap year occurs, a complication is introduced. February has then twenty-nine days. Traditionally, the Anglican and civil calendars added this extra day to the end of the month, while the Catholic ecclesiastical calendar counted 24 February twice. But in either case, 1 March is then one day later in the week than 1 February, 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, 1 January, 1907, was a Tuesday; the first Sunday fell on 6 January, or an F. F was, therefore, the Dominical Letter for 1907. The first of January, 1908, was a Wednesday, the first Sunday fell on 5 January, and E was the Dominical Letter, but as 1908 was a leap year, its Sundays after February came a day sooner than in a normal year and were D=92s. The year 1908, therefore, had a double Dominical Letter, E-D. In 1909, 1 January was a Friday and the Dominical Letter was C. In 1910 and 1911, 1 January fell respectively on Saturday and Sunday and the Dominical Letters are B and A.
This, of course, is all very simple, but the advantage of tile 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 I January, 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=92s, which gives the Dominical Letter for any year, or the second Dominical Letter if it be leap year:
For example, to find the Dominical Letter of the year 1913:
(Steps 1, 2, & 4) 1914 + 478 + 0 = 2392
(3) 19 - 16 = 3
(4) 2392 - = 2389
(5) 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 22 March or as late as 25 April, 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 1 January is a Sunday), Easter must be either on 26 March, 2 April, 9 April, 16 April, or 23 April, 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 days of the week on which 24 March 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 Concurrent 1 = F (Dominical Letter)
Concurrent 2 = E
Concurrent 3 = D
Concurrent 4 = C
Concurrent 5 = B
Concurrent 6 = A
Concurrent 7 = G