Cori Pringle, Allen Walton, Doug Davis

Friday the Thirteenth

Historically, people have been very superstitious when the thirteenth of any month falls on a Friday. Just the thought of this occurence strikes such fear in us that they created a series of horror movies of the same title.

Now, most people probably don't even know how many 13ths we can have on a Friday in one year. Using Microsoft Excel, I created calendars for every possibility (including leap years) and found how many times a year we needed to watch out for Jason.

For example, click here to see a calendar of a non-leap year with January 1st on a Sunday. Notice all of the Friday the Thirteenths are colored in red.

Now, I realize these aren't your typical calendars. But I have certain markings to let you know what is going on. As stated above, the thirteenths of any month that happen to fall on a Friday are colored in red. And, also, all of the firsts of the month are colored in a light green. As you look through each possible year, you can find how many Friday the Thirteenths are in each occurence. They are listed according to which day of the week January 1st will fall on.


Normal Year: Sunday / Monday / Tuesday / Wednesday / Thursday / Friday / Saturday

Leap Year: Sunday / Monday / Tuesday / Wednesday / Thursday / Friday / Saturday


We are currently in the year 2000, which is a leap year. As you should already know, leap years occur every four years and the difference is the additional day at the end of the month of February. January 1, 2000 fell on a Saturday and so, if you go to the choices and select Saturday under leap year, you will have a calendar of the year 2000. If you look at the friday column, you will find only one 13. This is the lowest possible number of friday the thirteenths in any given year. Now, look again through the rest and try to find the maximum possible friday the thirteenths.

If you look, you will find that in a non - leap year there are three friday the thirteenths only when January 1 is on a Thursday. In the leap years, the only possibility is the Sunday selection. So, the most we can have in one year is three but we can have no fewer than one. Pretty interesting. Let's look at some other information.

Let's take a look at the next ten years and see how many friday the thirteenths we can expect. The year 2000 ends on a Sunday, so January 1, 2001 will be on a Monday. Here below is each year and the calendar from 2000 to 2010.

2001 / 2002 / 2003 / 2004 / 2005 / 2006 / 2007 / 2008 / 2009 / 2010

2 / 2 / 1 / 2 / 1 / 2 / 2 / 1 / 3 / 1

Below the years are the number of friday the thirteenths you can expect in order. So, looks like 2009 will be the year for the spooks and we will have a total of 17 in this decade.

The last thing I want to look at is an odd occurence in the calendars. Let's look at a non-leap year that begin on a thursday. The year 2009 will actually be one of these years. Notice the months of February and March. Notice that the thirteenth of each month falls on a Friday. In no other year do two successive months have thirteenths on a Friday. Now, let's try to find out why.

Well, first off, we know that in a non-leap year February has 28 days and March has 31. So, if the thirteenth falls on a Friday in February, then there are only 15 days left in month. And, if we add thirteen to that we get 28. What do we know about 28? It happens to be a multiple of ...... 7! And, weeks are seven days long. Therefore, we know that four weeks from the thirteenth of February is the thirteenth of March. And, since the first one occurs on a Friday, they both do.

So, check your calendars and beware because you never know when you might have three scary days in one year.





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