Friday the 13th may be considered unlucky, but it's also more common than you might think. In fact, in the modern Gregorian calendar, it's more likely that the 13th day in any month will be a Friday than any other day of the week.
The fact that the number of days in a year doesn't perfectly match up with the Earth's rotational cycle has posed a problem throughout history. Pope Gregory's attempt to combat some of the complications of that problem caused riots in what has to be the first time battle in history.
A year is almost exactly 365 days...almost, but not quite. For millennia, people have tried to create calendars that account for that "not quite" bit. It's why we have leap days and why, just once, there was a February 30.