The Perennial Dominical Letter Calendar is a perennial calendar in which the Domincal Letters are used to name the days of the week. That is to say, the days of the 7-day week are A, B, C, D, E, F, and G. A-day is always 1 January and 31 December, and every date in between is invariably identified with a letter which is in alphabetical sequence from 1 January.
In leap years, February 29th is C-day, which is a repetition of February 28th. In this way, 1 March is always D-day.
So, in any given year, each date is always the same day of the week. A-day is repeated on December 31st, and January 1st, and C-day is repeated in leap years on February 28th and February 29th.
The Dominical Letter Calendar is an generalized extension of this calendar design.
— Walter Ziobro