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 Subject: calendar Category: Reference, Education and News > General Reference Asked by: maxconner-ga List Price: \$10.00 Posted: 13 Sep 2006 12:23 PDT Expires: 13 Oct 2006 12:23 PDT Question ID: 764964
 `how many times has October 13 fallen on a Friday?` Request for Question Clarification by pinkfreud-ga on 13 Sep 2006 12:27 PDT `What's the starting date? The Big Bang?`
 There is no answer at this time.

 Subject: Re: calendar From: fp-ga on 13 Sep 2006 12:31 PDT
 `Which calendar (e.g. Gregorian)? In which country?`
 Subject: Re: calendar From: myoarin-ga on 13 Sep 2006 13:37 PDT
 ```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.```
 Subject: Re: calendar From: barneca-ga on 13 Sep 2006 15:10 PDT
 ```for a sufficiently long period, it is very, very close to 1/7th of the time... -cab```
 Subject: Re: calendar From: myoarin-ga on 13 Sep 2006 16:14 PDT
 ```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.```
 Subject: Re: calendar From: barneca-ga on 13 Sep 2006 17:35 PDT
 ```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```
 Subject: Re: calendar From: myoarin-ga on 14 Sep 2006 14:57 PDT
 ```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.```