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, n° 32. The word is formed from dominica or dominicus dies, "Lord's-day, Sunday."
The dominical letters were introduced in the kalendar by the primitive Christians, in lieu of the SUNDIAL letters in the Roman kalendar.