Talk:Friday the 13th

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Semi-protected edit request on 21 November 2015[edit]

2601:201:8200:772A:29CB:A0FA:895A:647C (talk) 19:12, 21 November 2015 (UTC) Please add the St. Brice's day massacre into the text. 2601:201:8200:772A:29CB:A0FA:895A:647C (talk) 19:12, 21 November 2015 (UTC)

Cite error: There are <ref> tags on this page without content in them (see the help page).

  • Red question icon with gradient background.svg Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format.
  • You are going to have to be more specific as to where you want this and why you want this in the article. --Stabila711 (talk) 01:09, 22 November 2015 (UTC)
  • Add'l comment: The consensus has long been not to include events that happen to have occurred on Friday the 13th in this article (after all, one out of every seven 13ths falls on a Friday). There is a discussion in the topic immediately above to revisit this, but it does not look like the consensus is likely to change. So even of the IP editor is more specific about where he or she proposed to add the St. Brice's Day massacre, it's not likely to be added merely because it happened to have occurred on Friday, November 13, 1002. TJRC (talk) 22:31, 23 November 2015 (UTC)

Over an appropriate span of years, the 13th occurs exactly the same number of times on each day of the week[edit]

What I say below contradicts the conclusions of the author of the section in question. Therefore, I submit it for review. If a consensus of administrators agrees with me, then either I or, preferably, the original author can make the appropriate changes to the section.

In the section “Occurrence” the first paragraph (below the large table) begins: “This sequence, given here for 1900–2099, follows a 28-year cycle from 1 March 1900 to 28 February 2100.” The rest of the section then shows how “[t]he 13th day of the month is slightly more likely to be a Friday than any other day of the week.” I believe that this conclusion is incorrect, however, being solely the result of the choice of 200 years as the time span to be examined. It would also have been incorrect if the chosen time span had been 400 years.

The writer mentions the 28-year cycle but then ignores the crucial fact that in any 28-year cycle, each of the 365 common days (including each 13th) falls exactly four times on each day of the week. Since there are seven days in a week, each common day (including each 13th) occurs 7 × 4 = 28 times in 28 years (as it should), for a total of 28 × 365 = 10220 days; February 29 occurs seven times in 28 years, once on each day of the week. Thus in 28 years there ordinarily are 10227 (10220 + 7 = 10227) days.

This pattern holds for all spans of years that are integer multiples of 28 years and only for spans that are integer multiples of 28 years. It does not, therefore, hold for 200 years or for 400 years because neither is an integer multiple of 28 years (200 ÷ 28 = 7.14285714...).

On the other hand 420 years (or 392 years, and so on) is an integer multiple 28 years (420 = 15 × 28). Thus in 420 years the 13th of a given month falls on a Friday exactly 15 × 4 = 60 times, and the 13th of that month also falls exactly 60 times on each of the other six days of the week, as well. Thus the 13th of that month occurs 7 × 60 = 420 times in 420 years (as it should). The same is true for each of the 365 common days (including each of the other 13ths) giving a total of 365 × 420 = 153300 common days in 420 years. Adding in the nominal 105 occurrences of February 29 gives a total of 153300 + 105 = 153405 days in 420 years, which is exactly equal to 15 cycles of the 10227 days in 28 years (15 × 10227 = 153405).

To be perfectly accurate there are really only 153402 days in 420 years. That is because three of the four century years spanned have only 365 days, not 366. That means, as well, that three of the fifteen 28-year cycles (three of the four cycles that include a century year) have only 10226 days. Thus, recalculating, there are 12 cycles × 10227 = 122724 days and 3 cycles × 10226 = 30678 days for a total of 122724 + 30678 = 153402 days, which is the same as the true number of days in 420 years.

In addition, in 420 years there are 5040 months (420 × 12 = 5040). As said above, the 13th of a given month falls on a Friday exactly 60 times in 420 years. Since there are 12 months in every year, that means that there are 12 × 60 = 720 Friday the 13ths in 420 years. And, by the same reasoning, there are also 720 Thursday the 13ths and 720 Wednesday the 13ths, and so on. Since there are seven days in a week, the total number of 13ths in 420 years is 7 × 720 = 5040, which, as above, is exactly the number of months in 420 years.

The correct conclusion, therefore, is that in any span of 28 years or integer multiple of 28 years the 13th falls on each day of the week exactly the same number of times as it falls on every other day of the week. But that correct conclusion can only be reached by examining a number of years that is exactly an integer multiple of 28. In other words, by choosing to examine only multiples of 28 years, we discover that the 13th occurs on Friday exactly the same number of times as it occurs on every other day of the week.

If, for example, we examine 1200 × 28 years = 33600 years, we find the 13th of any given month occurring exactly 1200 × 4 = 4800 times on a Friday and also 4800 times on each of the other six days of the week, as well. Thus the 13th of each month occurs 7 × 4800 = 33600 times in 33600 years (as it should). (It is irrelevant and inconsequential that we are here not making the correction for the 252 century years that would have only 365 days instead of the 366 that otherwise occur every fourth year.)

And so it is misleading and leads to an incorrect conclusion to use a span of 400 years as the test span for deciding whether the 13th occurs on a Friday more often than it occurs on other days of the week. That number of years (400) is not evenly divisible by 28; the number is important only because, by adjusting the span of 400 years to have exactly 146097 days, we are easily able to keep the first day of spring close to March 21 over a span of many centuries before a more precise correction has to be made. The choice of 400 years had nothing to do with determining how many times the 13th fell on a Friday, as opposed to any other day. The choice of 400 years was made because it provided a convenient and simple rule-based calendar that was more accurate than the widely used Julian calendar.

In sum, only if you examine a span of years that is an integer multiple of 28 do you draw the correct conclusion that the 13th day of the month is exactly as likely to occur on a Friday as on any other day of the week. Wikifan2744 (talk) 01:46, 18 May 2016 (UTC)

@Wikifan2744: No, skipped leap days in century years do cause 28-year intervals to not have exactly 48 Friday the 13ths. For example, the ranges from 2045 to 2072, 2046 to 2073, 2047 to 2074, 2048 to 2075, 2049 to 2076, 2050 to 2077, 2051 to 2078, 2052 to 2079, 2053 to 2080, 2054 to 2081, 2055 to 2082, 2056 to 2083, 2057 to 2084, 2058 to 2085, 2059 to 2086, 2060 to 2087, 2061 to 2088, 2062 to 2089, 2063 to 2090, 2064 to 2091, 2065 to 2092, 2066 to 2093, 2067 to 2094, 2068 to 2095, 2069 to 2096, 2070 to 2097, 2071 to 2098, and 2072 to 2099 all contain 48 Friday the 13ths. In contrast, the ranges from 2073 to 2100, 2074 to 2101, 2075 to 2102, 2076 to 2103, 2077 to 2104, 2078 to 2105, 2079 to 2106, 2080 to 2107, 2081 to 2108, 2082 to 2109, 2083 to 2110, 2084 to 2111, 2085 to 2112, 2086 to 2113, 2087 to 2114, 2088 to 2115, 2089 to 2116, 2090 to 2117, 2091 to 2118, 2092 to 2119, 2093 to 2120, 2094 to 2121, 2095 to 2122, 2096 to 2123, 2097 to 2124, 2098 to 2125, 2099 to 2126, and 2100 to 2127 contain 48, 47, 47, 47, 46, 48, 48, 47, 48, 49, 47, 49, 49, 49, 49, 50, 50, 50, 49, 49, 50, 48, 50, 50, 48, 48, 49, and 47 Friday the 13ths respectively. GeoffreyT2000 (talk) 03:05, 11 August 2016 (UTC)

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Friday the 13th Has it ever been considered"Lucky"?[edit]

I know in some Hispanic nations 13 is considered a "Lucky" number and of course the foiunding of the United States had 13 colonies! Thanks! Eddson storms (talk) 00:56, 14 January 2017 (UTC)