Being an engineer, I am a math nerd so on Pi Day I usually make a pie to celebrate. Also, I like any excuse possible to make and eat pie. For non-math nerds, March 14 is Pi Day because the first few significant digits of the mathematical constant pi are 3.14. Pretty cute, huh? 
This year I am pretty busy with work before I leave for Italy tomorrow night so I sadly don't have time to make a pie but I thought I would share my pie from last year. I made a mixed berry pie with a double crust and had a lot of fun decorating the top crust.
Using a knife I cut out the symbol for pi, π, in the center of my crust and placed it on my pie. I then used mini fondant number cutters to make a border with the value of pi out to 36 significant digits (=3.14159265358979323846264338327950288). "Why 36 and not 35 or 37?" you may ask. Well, 36 digits is what fit on the pie. After cutting out the numbers I attached them to the crust with a little egg yolk brushed on the back. I finished the edge by trimming it, folding it under and then pinching it with my fingers to make it look pretty. Tasty + nerdy = awesome!
By the way, if anyone has any ideas for a fun baked good I can make to celebrate Mole Day on October 23, I am open to suggestions. Pi Day is easy, but Mole Day has me stumped...