If my memory is of any avail, week days repeat at regular
intervals of 18 years. Maybe I am wrong. Anyway, it is
easy to check if you have a perpetual calendar at hand.
It should be 28 years (for 7 days in a week x 4 years between leaps)
Specifically, there are 52 weeks plus 1 day in a year, so over 4 years the
day name advances by 5 days (one extra for the leap year). There are 7 days
in a week and the smallest multiple of both 5 and 7 is 35, so it takes 7
sets of 5 days to bring the name sequence back to where it started. That is
7 sets of 4 years, i.e. 28 years.
I assume this -28 rule doesn't normally work if you go through a century
boundary, because that would disrupt the leap year cycle. We were lucky that
2000 was a leap year.
Steve
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