FRIDAY THE 13TH
STRANGE BUT TRUE:
The 13th of the month is more likely to be a Friday
than any other day of the week!
Why?
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There are 14 different years, depending on which day of the week the year
starts on, and whether or not it is a leap year.
-
Leap years are those years divisible by 4, except for century years, which
must be divisible by 400.
-
The cycle of leap years thus takes 400 years to repeat.
-
There are 7 different cycles, depending on which day is the first day of the
cycle.
-
The number of days in a 400-year cycle turns out to be divisible by 7!
Thus, rather than cycling through the 7 different 400-year cycles, in which
case the 13th would be evenly distributed across all weekdays, we are locked
into a single cycle. In this cycle, the 13th lands on a Friday one more time
than on any other day of the week.
Tevian Dray
tevian@math.oregonstate.edu