from the GNU version of the Collaborative International Dictionary of English
- adj. the letter which, in almanacs, denotes Sunday, or the Lord's day (dies Domini). The first seven letters of the alphabet are used for this purpose, the same letter standing for Sunday during a whole year (except in leap year, when the letter is changed at the end of February). After twenty-eight years the same letters return in the same order. The dominical letters go backwards one day every common year, and two every leap year; e. g., if the dominical letter of a common year be G, F will be the dominical letter for the next year. Called also Sunday letter. Cf. Solar cycle, under Cycle, n.
Sorry, no etymologies found.