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Wednesday, January 19, 2011

Friday the 13th, part 2

As a continuation of the previous post, here's more.

1. Since the 13th occurs on Friday more often than on any other day, it's pretty obvious that the 12th would occur on Thursday more often than any other day, and so forth.

2. Because of the unevenness of months (28, 29, 30 or 31 days), there are months in the 400 year cycle in which the 13th does not occur most often on Friday. Again courtesy of Elroch, here's a "table of the number of times the 13th of each month is a particular day over the 400 year cycle, which should be enlightening. [Sorry, needs realigning - copying it to a spreadsheet should do it]."


    Mar    Apr    May    Jun    Jul    Aug    Sep    Oct    Nov    Dec    Jan    Feb   
Mon    56    58    57    58    58    58    56    57    56    56    57    58   
Tue    58    56    57    56    56    57    58    58    58    58    57    56   
Wed    57    58    58    58    58    57    56    56    57    56    58    58   
Thu    57    56    56    57    56    58    58    58    57    58    56    57   
Fri    58    58    58    57    58    56    57    56    58    57    58    57   
Sat    56    57    56    58    57    58    57    58    56    57    56    58   
Sun    58    57    58    56    57    56    58    57    58    58    58    56

So, over the 400 year cycle, there are 57 Friday the 13th days in June, but 58 Wednesday the 13th days.

Overall, if you are thinking "who cares?" you've got a point. I just found this interesting because I'd assumed that over a long period of time the numbers would end up being exactly 1 out of 7.