DOMINICAL LETTER (from Lat. Dominica, Sunday), properly called Sunday let ter, one of the first seven letters of the alphabet used in calendars to designate the Sundays throughout the year. The ancient Romans used the first eight letters. A—H, to denote their nundinal period of eight days; the early Chris tians dropped the eighth and adopted the re maining seven to indicate the days of the He brew week. These seven letters are employed to show on what days of the week the days of the month fall throughout the year. One of those seven letters must necessarily stand against Sunday, and is printed as a capital, and called the dominical letter or Sunday letter; the other six being inserted in different type, to denote the other six days of the week. Now, since a common Julian year contains 365 days, if this number be divided by seven (the number of days in a week), there will remain one day. If there had been no remainder it is obvious the year would constantly begin on the same day of the week; but since one remains, it is plain that the ordinary year must begin and end on the same day of the week; and therefore the next year will begin on the day following. Hence when January begins on Sunday, A is the dominical or Sunday letter for that year; then, because the next year begins on Monday, the Sunday will fall on the seventh day, to which is annexed the seventh letter, G, which therefore will be the dominical letter for all that year; and as the third year will begin on Tuesday, the Sunday will fall on the sixth day; there fore F will be the Sunday letter for that year.
Whence it is evident that the Sunday letters will go annually in retrograde order, thus, G, F, E, D, C, B, A; and in the course of seven years, if they were all common ones, the same days of the week and dominical letters would return to the same days of the months. But because there are 366 days in a leap-year, if the number be divided by seven, there will remain two days over and above the 52 weeks of which the year consists. And therefore, if the leap year begins on Sunday, it will end on Monday; and as the year will begin on Tuesday, the first Sunday thereof must fall on 6 January, to which is annexed the letter F, and not G, as in com mon years. By this means, the leap-year re turning every fourth year, the order of the do minical letters is interrupted, and the series can not return to its first state till after four times seven, or 28 years; and then the same days of the month return in order to the same days of the week as before. This method of indicating the days has held its place in Church calendars without variation from the beginning, and rules for finding them are given in breviaries, prayer books, etc.