how many times has October 13 fallen on a Friday?

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Request for Question Clarification by pinkfreud-ga on 13 Sep 2006 12:27 PDT

What's the starting date? The Big Bang?

Which calendar (e.g. Gregorian)? In which country?

Very astute, fp!  I expect that this is more than a $10 question.

Wikipedia provides interesting information, such as:
"The fear of Friday the 13th is called paraskavedekatriaphobia,
paraskevidekatriaphobia or friggatriskaidekaphobia, a specialized form
of triskaidekaphobia, a phobia (fear) of the number thirteen",
but no retrospective details.

for a sufficiently long period, it is very, very close to 1/7th of the time...

-cab

Barneca,
That is a little useful.  The wikipedia site says that Friday the 13th
occured 688 times in 4800 months, which is indeed extremely close to
every 7th month.  Someone more clever than I can calculate roughly how
often this 7th month was an October.

myoarin,

not sure if we're saying the same thing or not.  i'm coming at it from
another angle.  there is a complicated pattern due to leap years, but
in general, if the period is long enough, the pattern should smooth
itself out. 1/7th of the time, 10/13 would be a monday, 1/7th of the
time it should be a tuesday, etc., etc.

i just don't know if maxconner wants to know generally how common it
is (in which case i think i've answered the question), or for some
reason specifically wants to know exactly how many times it has
happened in a certain period, in which case i haven't.

-cab

Barneca,
I don't think we were, but a glance at the table in Wikipedia for the
years 2001-2028 says you are right  - if you meant on average once
every 7 years:

http://en.wikipedia.org/wiki/Friday_the_13th
The following months have a Friday the 13th:
2001 	2007 		2018 		April, July 	G
2002 		2013 	2019 	2024 	September, December 	F, GF
2003 	2008 	2014 		2025 	June 	E, FE
1992 			2020 		March, November 	ED
	2009 	2015 		2026 	February, March, November 	D
2004 					February, August 	DC
	2010 		2021 	2027 	August 	C
2005 	2011 	2016 	2022 		May 	B, CB
				2028 	October 	BA
2006 		2017 	2023 		January, October 	A
	2012 				January, April, July 	AG

It shows every month having a Friday the 13th four times in the 28 years.
(1992 slipped into the first column, so the line with March, November
counts as only one year, plus the three in the next line.)

Someone can figure out if this table of 28 years then repeats.  Things
will be a bit skewed by the rule for centuries, not leap years unless
they are divisible by 100 and also by 4.