Pi is the relationship of a circle's circumference to its diameter. No matter how large or small a circle is, the proportion of the distance around the circle to the distance across its widest part is EXACTLY the same!
That exact number starts off 3.14159265...and goes on for bajillions of non-repeating digits. Because pi starts with 3.14, we celebrate Pi Day on 3/14, or March 14th.
But today is not that date. Why are we celebrating Casual Pi Day today?
Well, the other name for the day gives us a hint—it's Pi Approximation Day.
It's hard to be exact about pi when the decimal digits go on forever (and don't even repeat). So we have to use approximate numbers when we actually use pi, and a really approximate way of expressing the approximation is the fraction 22/7. And that's why today, the 22nd day of the 7th month, is Pi Approximation Day!
Use a calculator to see what 22 divided by 7 is. How many decimal places match pi?
By the way, you can get a picture of pi being all casual and summery on a T-shirt or mug here.
When we are doing mental math or solving real-life problems in our heads, we often don't need the exact answer. Instead, we can use just an approximation of the right answer. And we do this by estimating—rounding off and THEN adding or multiplying or whatever we are trying to do...
Also on this date: