If you use the Julian or the Gregorian calender beware:

February of all the months of the year has only 28 days. Why so, is a question that crosses the mind once in a while, well... it is not a coincidence; it is a hint. If you take the extra days of the other months so that every month only has 28 days like February there will be enough days to creat a 13th month with... 28 days. Sure there is enough to have a month or two with 29 days, but there will be a 13th month in which you can have the 13th day of the 13th month. Good thing we have used a 12 month system for so long, because if it is the 13th day of the 13th month and you know it is that date: 'it' will happen.

(Note: It IS possible to learn the existence when such a date)