The number 13 is overloaded with superstitions. What’s worse than being 13 at the table? The origin of this idea is quite clear: it refers to the Last Supper (see above its representation in the church of Curahuara de Carangas in Bolivia), that is to say to the last meal of Jesus Christ where he designates the one who was to betray him and who will hang himself later. Even if the Gospels are more reminiscent of the 14th or 15th, some affirm that Jesus was crucified on Friday the 13th of the month of Nisan… which would thus be a day of misfortune. Yet for others, it is said to bring good luck. However, the statistics are terrible. If there are three times as many Loto players on Friday the 13th, their odds of winning remain exactly the same. Only the casino really benefits from Fridays 13.
13 months among the Maya
A quick reasoning might suggest that there are as many Fridays the 13th as there are Sundays the 13th or Mondays the 13th, etc. It is a mistake. A precise mathematical study of the Gregorian calendar shows that there are slightly more … which will undoubtedly delight the superstitious. The calculation is a bit laborious, we postpone it further for amateurs. To finish on the number 13, we can notice that, curiously, the Mayan sacred calendar had 13 months of 20 days each. This period is to be compared to the Mayan counting mode based on the base 20. The year thus comprised 260 days, which does not mean much from an astronomical point of view but that some approximate the duration of pregnancy, which is 266 days on average. Among the lucky numbers, we will cite 17 which is in Italy because XVII is the anagram of vixi which means “I lived” in Latin and therefore implies “I am dead”.
Number of Fridays 13
Since the Gregorian calendar reform of 1582, the years have been repeated identical every 400 years and not every 28 years as before in the Julian calendar. Indeed, if ordinary years always have 365 days and leap years 366, the rule to determine if a year is a leap has been modified: a year is if its vintage is divisible by 4 unless it is divisible by 100 but not by 400. The number of leap years in a period of 400 years is therefore 97 (and not 100) which gives 97 x 366 + 303 x 365 = 146,097 days… which is divisible by 7. Thus, January 1, 1600, was a Saturday, and likewise 400 years later, January 1, 2000. The year 2000 was identical to the year 1600. There was only one Friday the 13th in 1600 (in October) and therefore the same in 2000.
By counting the number of the thirteenth of the month over 400 years (which can be done by hand but more quickly by computer), we find 687 Sundays, 685 Mondays, 685 Tuesdays, 687 Wednesdays, 684 Thursdays, 688 Fridays and 684 Saturdays. The thirteenth of the month is therefore more likely to be a Friday than any other day of the week! Is this good news?
