SUNDAY LETTER. — In order to determine on which days of the year Sundays fell, medieval chroniclers devised the system of ferial letters, which involved marking the first seven days of the year with the first seven letters of the alphabet and then repeating the sequence every seven days until December 31.
Thus, the letter A corresponded to January 1, B to January 2, C to January 3, and so on up to the letter G, which marked January 7. The year was then designated by the letter corresponding to its first Sunday, known as the *littera dominicalis*. Once this was known, it was easy to deduce all the other Sundays. In leap years, however, there were two Sunday letters, since the addition of a day after February 24 meant that from that point onward the Sunday letter differed from the one that had applied from January 1 to that date. Due to the presence of leap years, the sequence of Sunday letters—which would otherwise repeat unchanged every seven years—actually recurs every 28 years; this period was known as the solar cycle (v.), so called because Sunday was called *dies solis*.