DOMINICAL LETTER. - In order to determine on which days of the year Sundays fell, medieval chronologists devised the system of ferial letters, whereby the first seven days of the year were marked with the first seven letters of the alphabet, and the series was begun again for every seven days until 31 Dec.
The letter A thus corresponded to 1 Jan., B to the 2nd, C to the 3rd, and so on up to the letter G, which marked 7 Jan.; the year was designated by the letter corresponding to its first Sunday, called littera dominicalis, once known which all the others could easily be deduced. In leap years, however, there were two dominical letters, since, with the addition of a day after 24 Feb., from that point onward the dominical letter differed from that applying from 1 Jan. to that date. Because of the presence of leap years, the series of dominical letters, which would otherwise recur unchanged every seven years, instead becomes identical again every 28 years; this period was distinguished solar cycle (v.), so called because Sunday was termed dies solis.