Math = Love: What day of the week were you born on?

Thursday, January 9, 2014

What day of the week were you born on?

Do you know what day of the week you were born on?  Do your students know what day of the week they were born on?

Yes, I know there's an app for that.  But, did you know that there is a mathematical formula that will tell you what day of the week you were born on?  If you were like me up until several weeks ago, the answer to that last question is a no.

So, how did I discover this?  I recently started a new classroom tradition.  Since September, we've been doing Good Things every Monday.  Sometime in October, I introduced Friday Funnies.  I stole this amazing idea from Lisa Henry.  I share jokes, comics, videos, or other random things that I found funny.  Yes, I torture my students with mathematical jokes sometimes.  They like to make fun of my jokes.  Apparently, I don't have a funny bone in my body.  But, if I forget that to do Friday Funnies, you would think that I had forgotten the password to the computer that has the capability of saving the world from destruction.  Okay, maybe I exaggerate a little.  My latest addition is Weird and Wacky Wednesday.  This is a chance to share something interesting and usually educational or inspiring with my students that they wouldn't otherwise be exposed to.

The Wednesday before Christmas Break, I was feeling uninspired regarding what I should share.  So, I googled "fun math facts" for inspiration.  After following a few links, I discovered a formula that can be used to determine what day of the week you were born on.  I already know that I was born on a Saturday, but I had to try out the formula to see if it worked.  And, it did!  It was magic; it was math!  I decided this would be the PERFECT thing to share with my kiddos.

It took a lot longer than I anticipated.  It's not that it's hard.  It's actually rather simple if you know how to do long division AND read directions.

What day of the week were you born on?
CAUTION: Only works for dates in the 1900s.  

I've decided that next year, I am going to save this activity for right before my Algebra 2 students jump into polynomial long division.  I always have to spend a good chunk of a class period reviewing long division with my students before we can even start to tackle long division with polynomials.  This activity provides the perfect reason for practicing long division.  You can't use a calculator for this because we are interested in the remainder.

I typed up the steps, but these steps only work for birthdays in the 1900s.  There is a modification to the formula (follow the link above and scroll down) that allows you to calculate the day of the week for dates in any century, but I have yet to experiment with it.  When I typed this up, I chose to use as much mathematical vocabulary as possible.  Vocabulary is one of the things I am really working on with my students this year.

Feel free to use this to amaze your students (if they were born in the 1900s) or try it out yourself just for fun!

After finding everybody's day of birth, we checked some of the dates using Wolfram Alpha to prove to students that the formula was true.  Much to my students' surprise, they were!  And, they had to admit.  It was pretty cool that you could figure it out using math!

PDF Template to Download


  1. Thank you for this post. I am teaching polynomial division on Monday, so I am going to start out with this activity as you suggested. I think it will be much more fun and exciting than my previous thought of a warm up with long division by itself. Thanks so much for your blog and the new ideas. I am a first year teacher and am learning a lot from you!

    1. I'm excited to hear how this works out for you! This was one of those activities that I tried out, THEN I realized that I should have saved it for a month or so until we got to rational functions. I guess hindsight really is 20/20. Please give us an update once you try it out!

      And, thanks for letting me know that you read my blog. That means so much to me!

  2. FUNNY FRIDAY!! My AP Calculus teacher has done this with us every Friday since the beginning of the school's something that everyone in our class looks forward to now! Happy Tuesday :)