DOMINICAL LETTERS or SUNDAY LETTERS, let ters employed in the construction of the calendar to mark the Sundays throughout the year. The first seven letters of the alphabet are taken to mark the first seven days of the year, the following sets of seven following on according to this marking. As the year consists of 52 weeks and one day the dominical letters go backward one day every year, the same order of letters recurring every 28 years, making the solar circle. See also CALENDAR.