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It would seem that, over a sufficiently long period, each day of the week should be equally likely. However, that is not the case. For the 400-year period, from 1901-2300 (which includes three non-leap year centuries and one leap year century), the average number of Friday 13th per year was 1.72 which is somewhat larger than the intuitive 1.71428571...

In fact Friday is the most common day of the week for a 13th. Of the 4800 13ths in this period,

  • 684 fell on a Thursday or on Saturdays (annual average 1.71)
  • 685 fell on a Mondays or on Tuesdays (annual average 1.7125)
  • 687 fell on a Sunday or on a Wednesday (annual average 1.7175)
  • 688 fell on a Friday (annual average 1.72).

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Q: How many Friday 13th on average in a year?
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