Answer:
Where you celebrate the math term PI by doing PI and eating it. (The food that is.)
If you ask me personally I think that it is really lame!
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National Pi Day is March 14. It's 3/14 -- get it?
Here is the official National Pi Day web site:
http://www.piday.org/
On March 14, 2015 at 9:26 it will be 3.1415926!! Cool!!