DOMINICAL LETTER, popularly called Sunday Letter, one of the seven letters A B C D E F G, used in almanacks to denote the Sundays throughout the year. (See CALENDAR, vol. vii, pp. 80, 81.) The word is formed from Dominica or Dominicus dies, the Lord's day. The dominical letters were introduced into the calendar by the primitive Christians instead of the mundane letters in the Roman calendar.