for National Geographic News
You're not having a nightmare. It really is Friday the 13th again.
For the first time in 11 years, Friday the 13th is falling in two consecutive months. This double threat can only occur in certain non-leap years and only in a February-March combination. Look for it—or avoid it—again in 2015.
The double whammy isn't the only Friday the 13th claim to infamy for 2009, a particularly tough year for superstitious minds.
The ominous date falls on three Fridays this year: February 13; this Friday, March 13; and again on November 13.
But three Friday the 13ths is the yearly maximum, as long as societies continue to mark time with the Gregorian calendar, which Pope Gregory XIII ordered the Catholic Church to adopt in 1582.
"You can't have any [years] with none and you can't have any with four because of our funny calendar," said Underwood Dudley, a professor emeritus of mathematics at DePauw University in Greencastle, Indiana, and author of Numerology: Or, What Pythagoras Wrought.
The calendar works just as its predecessor the Julian calendar did, with a leap year every four years. But the Gregorian calendar skips leap year on century years except those divisible by 400. For example, there was no leap year in 1900 but one was observed in 2000. This trick keeps the calendar in tune with the seasons.
The result is an ordering of days and dates that repeats itself every 400 years, Dudley noted. As time marches through the order, some years such as 2009 appear with three Friday the 13ths. Other years have two or one.
"It's just that curious way our calendar is constructed, with 28 days in February and all those 30s and 31s," Dudley said.
When the 400-year order is laid out, another revelation occurs: The 13th falls on Friday more often than any other day of the week. "It's just a funny coincidence," Dudley said.
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