DOMINICAL LETTER, popularly called Sunday-Letter, one of the seven letters A B C D E F G, used in almanacks, ephemerides, &c. to denote the Sundays throughout the year. See ASTRONOMY, n° 310. The word is formed from dominica or dominicus dies, Lord's-day, Sunday.
The dominical letters were introduced into the calendar by the primitive Christians, in lieu of the NUNDINAL letters in the Roman calendar.