February of all the months of the year has only twenty-eight days. Why this is so a question that crosses the mind once in a while. 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 twenty-eight days, like February, there will be enough days to create a thirteenth month with... twenty-eight days. Sure, there is enough to have a month or two with twenty-nine days, but there will be a thirteenth month in which you can have the thirteenth day. Good thing we have used a twelve month system for so long, because if the thirteenth day of the thirteenth month ever were to occur, you have to know that 'it' will happen.
(Note: It IS possible to learn if such an occurrence were to ever happen...)