"This is the Friday the Thirteenth Club, meeting in Paris in 1930 to dance underneath a ladder and carry open umbrellas indoors. Being a rational reader of Nature, you surely applaud this contemptuous attitude towards superstition, so you won't be at all concerned by the following sinister tale. In 1582, Pope Gregory XIII introduced a new calendar to replace the old Julian system, whose inaccuracies had made Easter slip slowly through the seasons. To bring the festivals back to their old positions, ten days disappeared from October 1582. Some people thought the days were being stolen from them. But rebuilding and resetting the calendar had a more subtle effect. The Gregorian cycle of 400 years contains exactly 20,871 weeks, and hidden in the calendar's machinery is a bias towards certain days of the week landing on certain dates in the month. The 13th is more likely to be a Friday than any other day (Brown, B. H. Amer. Math. Monthly 40, 607; 1933). Bernard Yallop now points out that with a personal computer it is possible to look for such peculiarities "without resorting to mathematics" (Spectrum October, 66; 1998). His table shows for example that there are 688 Friday-the-thirteenths every 400 years, but only 684 Thursdays; and a month (like a week) is most likely to begin on a Sunday. Did the Friday the Thirteenth Club know of their good fortune in having these extra opportunities to carouse? I only hope they didn't meet a sticky end before finding out."
Stephen Battersby, Nature, Vol 396, 12 November 1998, page 113