The New International Encyclopædia/Dominical Letter

​DOMIN′ICAL LETTER, or Sunday Letter. One of the seven letters A, B, C, D, E, F, G, used in calendars to mark the Sundays throughout the year. The first seven days of the year being marked in their order by the above letters in their order, then the following seven, and all consecutive sets of seven days to the end of the year, are similarly marked; so that the 1st, 8th, 15th, 22d, etc., days of the year are all marked by A; and the 2d, 9th, 16th, 23d, etc., by B; and so on. The days being thus marked, it is evident that on whatever day the first Sunday of the year falls, the letter which marks it will mark all the other Sundays in the year, as the number of the letters and of the days in the week is the same.

As the common year consists of fifty-two weeks and one day over, the dominical letters go backward one day every common year. If the dominical letter of a common year be G, F will be the dominical letter for the next year. As a leap-year consists of fifty-two weeks and two days, the letters go backward two days every leap-year. If in the beginning of a leap-year the dominical letter be G, E will be the dominical letter for the next year. This extraordinary retrocession, however, is made to take place at the intercalary day (the 29th of February) by the artifice of marking it by the same letter as the day preceding it, and thus the next Sunday is marked by the letter preceding that which marked the Sundays before the intercalary day. Suppose the 28th of February in a leap-year to be a Sunday, and marked by F, it is evident that the dominical letter for the rest of the year will be E. As every fourth year is a leap-year, and the letters are seven in number, it is clear that the same order of letters must return in four times seven, or twenty-eight years, which would, but for the leap-years, recur in seven years, and hence the solar cycle. (See Period.) The dominical letters were first introduced into the calendar by the early Christians, to displace the nundinal letters in the Roman calendar. They are of use as a means of discovering on what day of the week any day of the month falls in a given year. (See Easter.) Rules and tables for finding them are given in prayer-books, breviaries, etc., as well as in works on dates.

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