Dominical letters
The letters A, B, C, D, E, F, and G, as assigned to days in an ecclesiastical calendar ...
The letters A, B, C, D, E, F, and G, as assigned to days in an ecclesiastical calendar in order to aid in the determination of weekdays for a given year, as well as the date for Easter.
C.R. Cheney (ed.), Handbook of Dates for Students of English History (London: Offices of the Royal Historical Society, 1945), 8.