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 CHRONOLOGY, No 32. The word is formed from dominica or dominicus dies, "Lord's day, Sunday."
The dominical letters were introduced in the calendar by the primitive Christians in lieu of the NUNDINAL letters in the Roman calendar.