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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. 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.〔Peter Archer, ''The Christian Calendar and the Gregorian Reform'' (New York: Fordham University Press, 1941) p.5〕〔Bonnie Blackburn, Leofranc Holford-Strevens, ''The Oxford Companion to the Year'' (Oxford: Oxford University Press, 1999), p.829〕〔(Calendarium ) (Calendar attached to the papal bull "Inter gravissimas")〕 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.〔"Anno vicesimo quarto Georgii II. c.23" (1751), ''The Statutes at Large, from Magna Charta to the end of the Eleventh Parliament of Great Britain, Anno 1761'', ed. Danby Pickering, p.194.〕〔J. K. Fotheringham, "Explanation: The Calendar", ''The Nautical Almanac and Astronomical Ephemeris for the year 1931'', pp.735-747, p.745, ''... 1938'', pp.790-806, p.803.〕 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. == History == Per Thurston, (1909) dominical letters were: Thurston continues that the Christian Church, with its "complicated system of movable and immovable feasts" has long been concerned with the regulation and measurement of time; he states: "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."〔 He continues, that 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."〔 He then goes on to note that: and that this device was imitated by the Christians.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dominical letter」の詳細全文を読む スポンサード リンク
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