What is "dominical letter"

Dominical \Do*min"ic*al\, a. [LL. dominicalis, for L. dominicus belonging to a master or lord (dominica dies the Lord's day), fr. dominus master or lord: cf. F. dominical. See Dame.]

1. Indicating, or pertaining to, the Lord's day, or Sunday.

2. Relating to, or given by, our Lord; as, the dominical (or Lord's) prayer.
   --Howell.

   Some words altered in the dominical Gospels.
   --Fuller.

   Dominical altar (Eccl.), the high altar.

   Dominical letter, the letter which, in almanacs, denotes Sunday, or the Lord's day (dies Domini). The first seven letters of the alphabet are used for this purpose, the same letter standing for Sunday during a whole year (except in leap year, when the letter is changed at the end of February). After twenty-eight years the same letters return in the same order. The dominical letters go backwards one day every common year, and two every leap year; e. g., if the dominical letter of a common year be G, F will be the dominical letter for the next year. Called also Sunday letter. Cf. Solar cycle, under Cycle, n.

Dominical letters are letters A, B, C, D, E, F and G assigned to days in a cycle of seven with the letter A always set against 1 January as an aid for finding the day of the week of a given calendar date and in calculating Easter.

A common year is assigned a single dominical letter, indicating which letter is Sunday (hence the name, from Latin dominica for Sunday). Thus, 2011 is B, indicating that B days are Sunday, and by inference, 1 January 2011 is a Saturday. Leap years are given two letters, the first indicating the dominical letter for January 1 - February 28 (or February 24, see below), the second indicating the dominical letter for the rest of the year.

In leap years, the leap day may or may not have a dominical letter. In the original 1582 Catholic version, it did, but in the 1752 Anglican version it did not. The Catholic version caused February to have 29 days by doubling the sixth day before 1 March, inclusive, because 24 February in a common year is marked "duplex", thus both halves of the doubled day had a dominical letter of F. The Anglican version added a day to February that did not exist in common years, 29 February, thus it did not have a dominical letter of its own.

In either case, all other dates have the same dominical letter every year, but the days of the weeks of the dominical letters change within a leap year before and after the intercalary day, 24 February or 29 February.