Dominical Letter
Cyclopedia of Biblical, Theological and Ecclesiastical Literature [1]
the letter in our almanacs which marks the Lord's Day (Dies Domini), usually printed in a capital form. In the calendar, the first seven letters of the alphabet are applied to the days of the week, the letter A being always given to the 1st of January, whatsoever that day may be, and the others in succession to the following days. If the year consisted of three hundred and sixty-four days, making an exact number of weeks, no change would ever take place in these letters. Thus, supposing the 1st of January in any given year to be Sunday, all the Sundays would be represented by A; not only in that year, but in all succeeding. There being, however, three hundred and sixty-five days in the year, the first letter is again repeated on the 31st of December, and, consequently, the Dominical, or Sunday letter for the following year will be G. The retrocession of the letters will, for the same reason, continue every year, so as to make F the Dominical letter of the third, etc. If every year were common, the process would continue regularly, and a cycle of seven years would be sufficient to restore the same letters to the same days as before. But the intercalation of a day every bissextile or leap year causes a variation. The leapyear, containing three hundred and sixty-six days, will throw the Dominical letter of the following year back two letters; so that, if the Dominical letter at the beginning of the year be C, the Dominical letter of the next year will be A. This alteration is not effected by dropping a letter altogether, but by changing the Dominical letter at the end of February, where the intercalation takes place. In consequence of this change every fourth year, twenty-eight years must elapse before a complete revolution of the Dominical letter can take place; and it is on this fact that the period of the solar cycle is founded. The rules for finding the Dominical letter for any year are given in the Book of Common Prayer. (See Cycle).
The Nuttall Encyclopedia [2]
One of seven letters, A, B, C, D, E, F, G, used to mark the Sundays throughout the year, so that if A denote the first Sunday, it will denote all the rest, and so on with B, C, &c., till at the end of seven years A becomes the dominical letter again.
