The Perennial Dominical Letter Calendar by Walter Ziobro 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.

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