E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The letters which denote the Sundays or dis dominica. The first seven letters of the alphabet are employed; so that if A stands for the first Sunday in the year, the other six letters will stand for the other days of the week, and the octave Sunday will come round to A again. In this case A will be the Sunday or Dominical Letter for the whole year.