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From: ken@coho.halcyon.com (Ken Pizzini)
Subject: Re: Friday 13th, try it yourself (was Language Wars..)
Date: 1996/09/25
Message-ID: <52bmft$f3l@news1.halcyon.com>#1/1
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references: <51bv60$8d@pheidippides.axion.bt.co.uk>
 <uf3f0fmmrh.fsf@zen.microlise.co.uk> <3242D1EB.3F54@ford.com>
 <R.521pfp$kec@ns.ccsn.edu>
organization: What, me?
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 comp.lang.ada,comp.lang.apl,comp.lang.basic,comp.lang.c,comp.lang.fortran,comp.lang.perl.misc,comp.lang.pl1,comp.lang.rexx,comp.lang.pascal.misc,comp.lang.smalltalk
Date: 1996-09-25T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <R.521pfp$kec@ns.ccsn.edu>,
Russell Mosemann <mose@ns.ccsn.edu> wrote:
>   I decided to write a script of my own for Solaris 2.4 and perl
>5.003 which calls cal for 1066 to 1996 and rips out the Fridays.  The
>following is the number of times a particular day of the month falls
>on a Friday.  The results are pretty evenly distributed, i.e. it is an
>Urban Legend.

Try again, avoiding years befor 1752.  The cal program on Solaris
uses the Julian calender for years before 1752, the Gregorian
calendar for years after, and the calendar that England and
her colonies used for 1752 itself.  Under the Julian calendar
the 13th of a month will be equidistributed among each of the
days of the week, but under the Gregorian calendar you will find
that some days fall on the 13th more frequently than others
over a 400 year cycle.  (Note that any pair of dates on the
Gregorian calendar that are exactly 400 years apart will
fall on the same day of the week.)

		--Ken Pizzini