DOMINICAL LETTER, popularly called Sunday Letter, one of the seven letters A B C D E F G, used in almanacks, ephemerides, and the like, to denote the Sundays throughout the year. See CHRONOLOGY. The word is formed from Dominica or Dominicus dies, the Lord's day, or Sunday. The dominical letters were introduced into the calendar by the primitive Christians instead of the nundinal letters in the Roman calendar.