Computing the Number of Friday the 13^ths in a Year Using Excel

Quote of the Day

What is our intention for this meeting? What’s important? What matters?

— Oprah Winfrey. She starts every meeting with these three questions.

Figure 1: Number of Friday the 13ths By Year.

I have been tutoring math at the local library and using Excel as a vehicle for encouraging people to explore everyday math. While at the library, I heard a young man ask "How many Friday the 13^ths are in a year?" Since I am always looking for computational examples, I showed him how to use Excel to find the answer for himself. This post shows how I taught him to solve the problem. The solution turned out to be a good example of using Excel's date and array capabilities.

Figure 1 shows a simple table with the number of Friday the 13ths by year. I deliberately made the table 28 years per column to show that calendars repeat with a period of 28 years.  I use this characteristic of calendars to buy old calendars that I can reuse; I love old Christmas and tool company calendars.

Figure 2 shows the Excel array formula I used to compute the number of Friday the 13^ths. My workbook is here.

Figure 2: Formula Breakdown for Calculating the Number of Friday the 13^ths in a Year.