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"Any month starting with a Sunday will have a Friday the 13th" - Author Unknown

Friday, April 13, 2012 & the Calendar

Friday the 13th three times over in 2012

Yes, the year 2012 gives us a Friday, April 13. But that's only a footnote in this year's epic Friday the 13th drama. Exactly thirteen weeks previous to April 13, it was Friday, January 13. And exactly thirteen weeks later, it'll be Friday, July 13. Yes, it's Friday the 13th three times over in 2012!

2012 Calendar28-Year Calendar Cycle

It's inevitable. Any leap year with a Friday, January 13, must also have a Friday, April 13, and a Friday, July 13. This is the only scenario that allows for three Friday the 13ths to be spaced at exactly thirteen weeks apart in the same calendar year.
Three Friday the 13ths also happen in a non-leap year whenever there's a Friday, February 13. In this case, it's inevitable that a Friday, March 13 and a Friday, November 13, should follow.
Three years before and after 2012 - 2009 & 2015 - present February-March-November Friday the 13ths. And fourteen years before and after 2012 - 1998 & 2026 - give us February-March-November Friday the 13ths as well.
Months and years having Friday the 13th
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As a general rule, you can count on any particular kind of Friday the 13th year to repeat every 28 years. That's because the days of the week faithfully realign with the same calendar dates in 28-year cycles.
Sure enough, the last triple Friday the 13th leap year took place in 1984, and will happen again in 2040. In the 21st century (2001-2100), we can expect January-April-July Friday the 13ths to happen in 2012, 2040, 2068 and 2096. But how about the year 2124? The answer is no!

Suppression of Leap Year Alters 28-Year Cycle

If a leap year of 366 days always took place every four years, ad infinitum, then we'd see the repetition of the January-April-July Friday the 13th cycle every 28 years, ad infinitum. But alas, by Gregorian calendar rules, any (end of the) century year not equally divisible by 400 is not a 366-day leap year. That means the years 2000 and 2400 are 366-day leap years, but that the years 2100, 2200 and 2300 are common years of 365 days.
Therefore, the suppression of the leap year in the year 2100 will "perturb" the cycle in the 22nd century (2101-2200). Thus, the first Friday the 13th January-April-July "tripleheader" of the 22nd century will take place in 2108, instead of 2124. Yet the 28-year cycle will work within the confines of the 22nd century: 2108, 2136, 2164 & 2192.

Suppressed Leap Years of 2220 and 2300 "Perturb" Cycle in 23rd and 24th Centuries.

In the 23rd century (2201-2300), the first leap-year January-April-July trio occurs in the year 2204 instead of 2220. Therefore the succession in the 23rd century (2201-2300): 2204, 2232, 2260 and 2288.
In the 24th century (2301-2400), the first leap-year trilogy falls in 2328 instead of 2316. The succession in the the 24th century (2303-2400): 2328, 2356 and 2384.

Cycle Repeats Every 400 Years

Because the cycle repeats every 400 years, the years in the 25th century (2401-2500) will match those of the 21st century (2001-2100): 2012, 2040, 2068 and 2096. Simply add 400 years! Thus ends our abbreviated tale of Friday, April 13, 2012 & the Calendar. . .

copyright 2012 by Bruce McClure

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