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Prof. Underwood Dudley Offers Fun Trivia About Friday the 13th

January 29, 1998

January 29, 1998, Greencastle, Ind. - Friday the 13th has an interesting role in our calendar, according to Underwood Dudley, DePauw University professor of mathematics and author of two books, including a recent one on numerology. The Gregorian calendar, which is currently in use, operates on a 400-year cycle so that years repeat themselves exactly during each 400-year period. Thus, 1998 has the same days as 1598 did in some of the world at that time, Dudley said. The Gregorian calendar was not introduced to the American colonies until 1752.

Dudley also offers the following trivia about Friday the 13th:

  • The 13th of a month is more likely to fall on a Friday than on any other day.
  • In each 400-year cycle of the Gregorian calendar, 170 years have one Friday the 13th, 171 have two, and 59 have three (including 1998).
  • It is impossible to have a year with no Friday the 13th or a year with four or more Fridays the 13th.
  • When there are three Fridays the 13th in a year, they occur in February, March and November (including 1998) except in leap years. In leap years, they occur in January, April and July.Friday 13th Calendar.jpg
  • When there is only one Friday the 13th in a year, it falls either in May, June, August or October, which are not generally thought of as being unlucky months.
  • The longest period of time with no Friday the 13ths is 426 days. The shortest period with four Friday the 13ths is 428 days.
  • And, by the way, what became known as the Gregorian calendar was ordered in 1582 by Pope Gregory the Thirteenth.