Let us first assume that you are only interested in which dates fall
on which days of the week; you are not interested in the dates for
Easter and other irregular holidays.
Let us further confine ourselves to the years 1901-2099.
With these restrictions, the answer is as follows:
- If year X is a leap year, you can reuse its calendar in year X+28.
- If year X is the first year after a leap year, you can reuse its
calendar in years X+6, X+17, and X+28.
- If year X is the second year after a leap year, you can reuse its
calendar in years X+11, X+17, and X+28.
- If year X is the third year after a leap year, you can reuse its
calendar in years X+11, X+22, and X+28.
Note that the expression X+28 occurs in all four items above. So you
can always reuse your calendar every 28 years.
But if you also want your calendar's indication of Easter and other
Christian holidays to be correct, the rules are far too complex to be
put to a simple formula. Sometimes calendars can be reused after just
six years. For example, the calendars for the years 1981 and 1987 are
identical, even when it comes to the date for Easter. But sometimes a
very long time can pass before a calendar can be reused; if you happen
to have a calendar from 1940, you won't be able to reuse it until the
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