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How Many Friday the 13ths

Date: 9/12/96 at 21:15:31
From: Jim Smith
Subject: How Many Friday the 13ths

Is there a way to figure how many Friday the 13ths can occur in a 
given year?

Date: 9/13/96 at 12:59:5
From: Doctor Tom
Subject: Re: How Many Friday the 13ths

There's no clean, simple way to do this, since there's very little
mathematical structure to the calendar - it's mostly political.

For example, did you ever wonder why February got short-changed on
days when there were plenty of months to "steal" the 31st day from?

Well, Augustus Caesar didn't like the fact that Julius' month (July)
had 31 days and his (August) had only 30, so he "stole" a day from
February to make August have 31.

You can get an answer to your problem, however. There are exactly
14 different calendars possible. January 1 can be any one of 7
days, and if you know whether there's a February 29th, you can
work out what days are Friday the 13th for each of them.  I've
even seen "universal calendars" that consist of 14 copies of a
calendar for a year and then a list of years that tells you
which calendar to use.

-Doctor Tom,  The Math Forum
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