2015 has three Friday the 13s. What are the odds?

[Panich52]

2015 has three Friday the 13s. What are the odds?
Feb 11, 2015
by Bruce McClure in Blogs » Human World

EarthSky
http://earthsky.org/human-world/three-friday-13ths-in-2015-february-13-march-13-november-13?utm_source=EarthSky+News&utm_campaign=658e6e8a01-EarthSky_News&utm_medium=email&utm_term=0_c643945d79-658e6e8a01-393525109


Scary coincidence or super unlucky? Neither. Here’s the story behind why 2015 has three Friday the 13ths.

Scared of Friday the 13th? An irrational fear of this date is called friggatriskaidekaphobia (Frigga is the Norse goddess for whom Friday is named).

February 13, 2015 is a Friday. And that’s just the beginning of this year’s epic Friday the 13th trilogy. We’ll have a Friday the 13th in March, too – exactly four weeks after February’s Friday the 13th! Then we’ll have a Friday the 13th in November – exactly 39 weeks (3 x 13 weeks) after February’s Friday the 13th!

Not that we at EarthSky suffer from friggatriskaidekaphobia – an irrational fear of Friday the 13th – but, gosh darn, it’s Friday the 13th three times over in 2015. Follow the links below to learn more about why some people fear this day and about 2015’s three Friday the 13ths.

Folklorists say there's no written evidence that Friday the 13th was considered unlucky before the 19th century. The earliest known documented reference in English appears to be in Henry Sutherland Edwards' 1869 biography of Gioachino Rossini, a 19th century Italian composer.

Scary coincidence or super unlucky? Neither. It’s just a quirk of our calendar, as you’ll see if you keep reading.

The fact is that, according to folklorists, there’s no written evidence that Friday the 13th was considered unlucky before the 19th century. The earliest known documented reference in English appears to be in Henry Sutherland Edwards’ 1869 biography of Gioachino Rossini. His portrait is on this page. He doesn’t look scary.

Friday has always gotten a bad rap. In the Middle Ages, people would not marry – or set out on a journey – on a Friday.

There are also some links between Christianity and an ill association with either Fridays or the number 13. Jesus was said to be crucified on a Friday. Seating 13 people at a table was seen as bad luck because Judas Iscariot, the disciple who betrayed Jesus, is said to have been the 13th guest at the Last Supper. Meanwhile, our word for Friday comes from Frigga, an ancient Scandinavian fertility and love goddess. Christians called Frigga a witch and Friday the witches’ Sabbath.

In modern times, the slasher-movie franchise Friday the 13th has helped keep friggatriskaidekaphobia alive.

In 2015, blame Thursday. The year 2015 started on a Thursday. Whenever a common year of 365 days starts on a Thursday, it’s inevitable that the months of February, March and November will start on a Sunday. And any month starting on a Sunday always has a Friday the 13th.

Of course, February has exactly four weeks in a non-leap year. So, for that reason, the days of the week have to match up with the same dates in both February and March during any common year. In any year, the days of the week always fall on the same dates in both March and November. In short, because the year 2015 started on a Thursday, that means February, March and November all have to start on a Sunday and all must have a Friday the 13th.

The February-March-November Friday the 13th trilogy repeats … More often than you might imagine! The last February-March-November Friday the 13th year happened six years ago, in 2009, for the first time in the 21st century (2001-2100). It will next happen eleven years from now, in 2026. After that, the following February-March-November Friday the 13th year will happen eleven years after 2026, in the year 2037.

A grand total of eleven February-March-November Friday the 13th years takes place in the 21st century:

2009, 2015, 2026, 2037, 2043, 2054, 2065, 2071, 2082, 2093 and 2099.

The rhyme and reason of the Friday the 13th cycle. Yes, it does make sense. Within the 21st century (2001-2100), note that the February-March-November Friday the 13th years repeat in 28-year cycles (going crosswise):

2009, 2037, 2065, 2093
2015, 2043, 2071, 2099
2026, 2054, 2082

Because the Gregorian calendar suppresses the leap year in 2100, the cycle is perturbed, meaning that all eleven February-March-November Friday the 13th years in the 22nd century (2101-2200) come four years earlier than in the 21st century:

2105, 2111, 2122, 2133, 2139, 2150, 2161, 2167, 2178, 2189 and 2195.

However, within the 22nd century (2101-2200), these Friday the 13th years also repeat in cycles of 28 years.

2105, 2133, 2161, 2189
2111, 2139, 2167, 2195
2122, 2150, 2178

Friday-the-13th-year repetitions within 28-year cycle. Some of you, who might not yet be dazed by calendar numerology, may wonder if some formula governs how a given Friday the 13th year repeats within the 28-year cycle. The answer is a definite yes. Keep in mind that this particular February-March-November Friday the 13th year can only happen in a common year of 365 days, and when January 1 falls on a Thursday.

Therefore, if this threefold Friday the 13th year comes one year after a leap year, the days again match up with the dates in 6, 17 and 28 years afterward. Take the year 2009, for example, which comes one year after a leap year:

2009, 2015, 2026, 2037

However, if this triple Friday the 13th year falls two years after a leap year, the days and dates realign in 11, 17 and 28 years. Take the year 2026, which takes place two years after a leap year:

2026, 2037, 2043, 2054

Finally, if this trio of Friday the 13ths happens three years after a leap year, the days recur with the same dates in 11, 22 and 28 years. The year 2015 happens three years after a leap year:

2015, 2026, 2037, 2043

Can three Friday the 13ths occur in a leap year?
Yes, a leap year can harbor three Friday the 13ths (January 13 – April 13 – July 13) if the leap year starts on a Sunday, which last happened in 2012. However, given that this particular Friday the 13th year happens in a leap year, and a leap year only, it recurs only in periods of 28 years. So the last January-April-July Friday the 13th year happened in 1984, and will next happen in 2040.

If a common year starts on a Thursday, there are three Friday the 13ths; and if a leap year begins on a Sunday, there are three Friday the 13ths. These are the two scenarios whereby three Friday the 13ths can occur in single calendar year.

Bottom line: Scary coincidence or super unlucky? Neither. Here’s the story behind why 2015 has three Friday the 13ths.


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[Kalidurga]

1. A math wiz can help me out on this

there are 7 days in a week, 12 months in a year. Mathmatically I think that means each day of the week will be represented about 1.6 times. So, a Friday occuring 3 times is a bit unusual I would think, but I have no idea what the odds are from that 25% maybe?

[shenmue]

2. Oh noes. I skeered

Mommy