13

The number 13 is widely regarded as an unlucky number. The reason for this is almost certainly due to the use of the number 12 in many mathematical exercises, for example that there are 12 hours on the clock (then repeated for AM and PM) and thus 13 o'clock was considered to be unlucky, because it should never happen. Similarly, in the days before playing cards were created, dice games were popular, and, whilst 1-dice games were popular, also 2-dice games would have been popular. Since the maximum possible total is 12, if you ever got 13, then it would be considered to be terribly unlucky. Of course, the only way to get 13 is if someone didn't add the total correctly, which likely would have happened quite a lot (for example 5+6 might be mistaken to equal 13 instead of 11) as mathematical skill was rather poor in the days of dice game popularity.

The paranoia of the number 13 spread so far as to the calendar. For thousands of years, most primitive societies (including the incas, ancient greeks and ancient egyptians) used a 28 day month, based on the cycle of the moon. This month was typically split into 4 quarters, often called weeks, each of 7 days duration. There are, almost exactly, 13 x 28-day months in the year (this results in 364 days, as compared to 365 1/4 days in the actual year). If we were to use a 13 month calendar, it would be a more accurate calendar and each month could be the length of an actual moon cycle, with 1 special day per year, plus, every 4th year, a 2nd special day per year.

The primary reason that 13-month calendars were never introduced (in any society, ever) was because of the superstition surrounding the number 13. It was considered that the number 13 was too unlucky, and as such 12-month and of course briefly 10-month calendars were introduced, with varying numbers of days in each month. Whilst 28-day months were tried by the romans (with 10-month years), this was, predictably, an abysmal failure. Decimalising time has never worked particularly well.

13 continues to be considered in most cultures to be an unlucky number. The time between midnight and 1am is often referred to as the "witching hour", based on the superstition that 1am meant 13 o'clock. As such, it was considered to be a terribly bad idea to be outside between midnight and 1am, as that was supposed to be when bad things happened.

Many flights, especially international flights, refuse to have a 13th aisle in their planes, because of this superstition, and in almost every society on earth the number 13 is to this day considered to be unlucky.